Two new studies link parents' smoking to cancer and serious infectious diseases in children, and one suggests that the children's cancer may arise from the harmful effects of smoking on fathers' sperm.

In one study, researchers 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 the study's principal author, Anne T. Berg of the Yale University School of Medicine.

A separate study showed that men who smoke have an increased risk of fathering children with brain cancer and leukemia, suggesting that smoking might have harmed the men's sperm, researchers said Wednesday.

That conclusion is speculative, but the implication is strong enough that "another study with bigger numbers ought to look at it carefully," said one of the study's authors, Dale P. Sandler of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C.

Both studies appear in the current issue of the American Journal of Epidemiology.

The link between parents' smoking and serious infections was a surprise finding from a study designed to see if attendance in day-care centers increased children's risk of getting infectious disease.

The study found that, with one exception among 386 children studied - a case of bacterial meningitis, previously known to be transmitted in such settings - those in day care were not more likely to develop serious diseases. But researchers discovered that "children who were hospitalized were more likely to live with a smoker than were children who were not hospitalized," Berg said in an interview. The risk of serious infection to children who lived with a smoker was three to four times higher, she said.

Berg said the variety of infections included digestive-system infections and respiratory infections, possibly because smoke depressed the children's immune systems generally.

The study that raised questions about the effects of smoking 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.

They found that the risks of leukemia and lymphoma were 30 percent higher in children whose mothers smoked during pregnancy, compared with children whose mothers didn't smoke.

They also found an increased risk of leukemia, lymphoma and brain cancer in children whose mothers didn't smoke but whose fathers did.

"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," John said.

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

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.