ONCE AGAIN suburban Marylanders are being asked to bail out a badly planned hospital. The latest supplicant is Shady Grove Adventist Hospital in Gaithersburg, which is scheduled to open this fall and faces the prospect of few patients and large deficits for the first few years.

Shady Grove's woes stem largely from overoptimism of several kinds. It was planned as part of a new complex of medical buildings, but the others are not ready yet. It was launched at a time when county and state health-care planners expected the Gaithersburg area's population to soar. That has not happened. Shady Grove's sponsors also promised that its alliance with Washington Adventist Hospital in Takoma Park would produce substantial cost savings. Those gains have turned out to be smaller. Finally, the hospital's officials were so eager to proceed that when a legal challenge by a competing hospital delayed their permanent financing, they went ahead with short-term, higher-cost loans.

So now they have their hospital - and it is starting out with large debts and lots of empty beds, perhaps 50 percent. At the room rates that Maryland regulators would normally authorize, around $116 a day, Shady Grove's deficits could add up to $5 million in the first two years. The hospital's officials are therefore asking the state rate-review commission for higher charges, around $130 per day. They are also hoping that Montgomery County will come up with subsidies until the new medical complex gets into full swing.

Both requests should be turned down. Hiking the rates would shift the hospital's losses to its patients - and to their insurance companies, and thus to everyone who pays health-insurance premiums in the area.

A county subsidy would put the costs on the taxpayers - who are, of course, largely the same group. And either kind of subsidy would reinforce the assumption, already too well established in Maryland, that hospital builders and investors don't have to worry too much about efficiency or good planning because premature and overly expensive projects will be propped up. If Shady Grove cannot pay its way at the normally justified rates, the hospital's sponsors and the backers of the new medical complex are the ones who should have to pay.