NEW YORK -- Fathers who smoke increase their risk of having children with brain cancer and leukemia, and this suggests that smoking may damage the fathers' sperm, according to a new study.

That conclusion is speculative, said Dale P. Sandler of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C., who helped write the study.

But the study at least raises the possibility of an effect on sperm, "and another study with bigger numbers ought to look at it carefully," she said.

In a separate study, doctors found that children whose parents smoke are three to four times as likely as other children to develop serious infectious diseases requiring hospitalization.

"I don't think anyone before has demonstrated that the association is not just for mild illnesses, but for really serious infections as well," said Anne T. Berg of the Yale University School of Medicine, principal author of the study.

The two studies appear in the current issue of the American Journal of Epidemiology.

The study that raised questions about smoking's effects on men's sperm also found that mothers' smoking could lead to an increased risk of cancer in children.

Sandler, Esther M. John of the Stanford University School of Medicine and David Savitz of the University of North Carolina studied 223 children with cancer and 196 children without cancer who were selected as a control group.

The researchers found that the risks of leukemia and lymphoma were 30 percent higher in children whose mothers smoked during pregnancy.

They also found that children exposed only to their father's smoking before birth had an increased risk of leukemia, lymphoma and brain cancer.

"A distinguishing feature of our study is that we found fathers' smoking to be an independent risk factor," John said. "If the association with fathers' smoking is confirmed in future studies, it may suggest a genetic effect on the sperm cells caused by the fathers' smoking," she said.

The increased cancer risk might also be due to the children's exposure to fathers' cigarette smoke after birth, Sandler said.

"One thing nobody's started to look at is whether all this smoking over the past 80 years or so has affected the whole genetic pool," said A. Judson Wells, a volunteer with the American Lung Association and an authority on passive smoking. "There are worries about this."

John and her colleagues estimated that about 6 percent of all childhood cancers and perhaps 17 percent of cases of acute lymphocytic leukemia might be due to mothers' smoking.

The other study, linking parents' smoking to serious infections, was designed to see if attendance at day-care centers increased children's risk of getting infectious disease. (It generally did not.) But the researchers found something they were not looking for.

"We did find that children who were hospitalized {for infectious diseases} were more likely to live with a smoker than were children who were not hospitalized," Berg said.

The children had an increased risk of a variety of infections, including digestive system and respiratory infections, Berg said. That could mean that smoking is depressing the children's immune systems generally, allowing infections to take hold more easily, she said.