"Paternal smoking seems to be real" as a risk factor, said Patricia Buffler, a professor at the University of California, Berkeley, who was not involved in the current analysis.

The research team, led by Dr. Elizabeth Milne at the Telethon Institute for Child Health Research in Australia, surveyed the families of nearly 400 children with acute lymphoblastic leukemia (ALL).

Although ALL is the most common childhood cancer, it is still rare, affecting about three to five children out of every 100,000, according to the National Cancer Institute. More than 1,000 kids die of the disease every year.

The survey asked about the smoking habits of both parents.

Milne and her colleagues compared these families to the families of more than 800 children of similar ages who did not have leukemia.

They found that the mothers' smoking behaviour had no impact on the kids' risk of developing the cancer.

But kids whose fathers smoked at all around the time of their conception were 15 per cent more likely to develop leukemia. Those whose dads smoked at least 20 cigarettes per day around that time were 44 per cent more likely to be diagnosed with the cancer.

A 15 per cent increase in the risk of ALL would increase the number of cases from six out of every 200,000 children to seven out of every 200,000.

Of the nine earlier reports that the researchers used in their comparison with the current study, six of them also found an increased risk.

The findings make sense, Buffler said. "Tobacco smoke is full of toxins," including carcinogens, she said, "so it's not unlikely that you'd have damage" in the cells that produce sperm.