Shortly after 1 a.m. recently, on-call in the psychiatric emergency room of a Boston hospital, I was asked to evaluate a homeless man and in the process, confronted the limits of my professional empathy.

My beeper had woken me after 20 minutes of sleep, and the emergency room nurse at the Boston Veterans Administration Hospital handed me a consultation form on which she had written the identifying information: 56-year-old, divorced, homeless veteran complaining of depression; history of alcohol dependence, no apparent suicidal or homicidal impulses.

The negatives surprised me a bit. Many homeless veterans who know the VA hospital system and need a place to sleep have learned to claim emphatically that they are feeling suicidal or homicidal.

Hospital beds are at such a premium that any psychiatric problem short of life and death usually is made to wait until normal business hours.

Against a constant flow of would-be inpatients, one of my roles as a resident physician is that of gatekeeper, evaluating the veracity of suicidal or homicidal threats, attempting to defuse empty threats in order to justify keeping the ward census at a reasonable number.

After nights when I have not admitted any patients, I have been congratulated by colleagues, slapped on the back and affectionately called a "wall."

But this man was a newcomer to the emergency room; he didn't know how to manipulate the system. He had only recently lost his job, then his wife, then his home. For a month he had wandered, drinking to forget, sleeping in shelters. But tonight he had sobered up too late to secure a shelter bed, and, alone in the freezing wind, he felt his grief and exhaustion weighing more heavily than ever.

"I need to be in the hospital," he told me. "I have to get a handle on myself."

I listened at length to his description of the losses he had suffered. "Have things gotten so bad for you that you've thought of hurting yourself?" I asked.

"I would never do that," he replied.

"Some people get so angry that they start thinking of hurting someone else."

"Look, doc, I'm worn out. Period. I'm not mad at anyone but myself."

He denied symptoms of clinical depression or any other major mental illness. His intellect and memory were normal. He had never been admitted to a psychiatric hospital and took no medication for emotional problems. As if to offer something to reward my search for symptoms, he showed me his feet, skinned and bloody from walking the streets.

"There's no question you need help with the problems you've talked about," I said. "I can help you follow up with the outpatient clinic downtown."

He took the slip of paper on which I had written the clinic's address and phone number. "I don't need to be in the hospital?" he asked, plaintively.

"No," I said. "But I'd like to be sure you'll make an appointment with the clinic. They may decide to schedule an admission in the future."

"I will," he nodded. He looked at me expectantly. "Where do I go now?"

Sometimes the answer to that question is simple, but not this time. None of the shelters the night hospital staff called had any beds available. The admitting office of the hospital had no room to house another patient on a ward. The security guard reluctantly reminded me that no one was allowed to sleep overnight in the hospital lobby. The buses to the airport, where homeless people have told me they sometimes find safe corners, had stopped running. The hospital's petty cash fund had run dry.

"I can't find a place anywhere for you," I said, after nearly an hour, shaking my head.

"It's cold," he said. "It'll be four, five hours before it's light."

I reached into the pocket of my scrubs and handed him three dollar bills. "Maybe the subway?" I suggested.

He started putting his socks over the medicated gauze I had wrapped around his injured feet. "The subway's dangerous," he said.

I stood up. "I wouldn't want to be there myself, but there's not a lot more I can do."

That was a lie, of course, and I believe we both knew it. I'd only part with three dollars for a person in no immediate danger with nowhere to go.

The two twenties upstairs in my call room -- enough for a taxi and motel -- stayed there. My car, good shelter from the wind, was parked right out front. I didn't even think of unlocking it for him. I have family and friends who live not 30 minutes from the hospital, with extra beds. I wouldn't think of asking them to open their homes to a stranger. And as a psychiatrist, empathy is my calling.

Why wouldn't I do more? The reason is that I had hit my internal "wall." Part of it was fear. I have learned that I really don't know much about anyone after an hour.

Moreover, despite all my attempts to banish it, I still harbor the prejudice that those who cannot sustain themselves in society are less likely to be bound by society's rules. Losing all one's possessions raises the suspicion that a person is somehow out of control in every way.

I felt -- unfairly or not -- that to get involved with this patient would put me at risk of being physically harmed or at least exploited. Maybe I was afraid of being overwhelmed, that if I truly extended myself to one homeless man, what would prevent my being used up by the sheer bulk of homeless people?

As a child, I was more than once admonished by a teacher not to share candy with a friend if I didn't have enough for everyone. So I kept it to myself.

I believe I also had myself convinced that my restraint had a therapeutic component. Perhaps, I thought, this man had not yet fallen far enough to take hold of himself, to stop drinking, to get another job. And even if he didn't spend the money on booze, a room tonight might be no more than a bandage obscuring an infected sore, allowing whatever infection was at work to do more lasting damage. What, indeed, if his disorder was a dependent personality? I would be playing into his pathological inability to be self-sufficient.

The problem is that these are theories.

I know there are many people who need to be taken care of. But at 2 a.m. the job of distinguishing those who need firm limits from those who need warm beds from those who need to be left alone is overwhelming.

So I am left with the nagging guilt that I could, or should, have done more for this man.

Keith Russell Ablow is a writer and a senior resident in psychiatry at New England Medical Center in Boston.