Babies absorb substantial levels of cancer-causing chemicals if their parents smoke, say scientists.
Half of babies with smoking parents have worrying levels of carcinogens in their urine.
Researchers said this suggests that passive smoking in childhood could be linked to cancer in later life.
The US study adds to the evidence that passive smoking can harm babies. It involved 144 babies from homes where at least one parent smoked.
A team from the University of Minnesota's Cancer Centre in Minneapolis tested urine from each child for the chemical NNAL, which is created as the body breaks down a carcinogenic chemical found in tobacco.
The study found that 47 percent of the children had detectable levels of NNAL in their urine.
Typically, the parent got through 76 cigarettes a week in their home or car while their baby was present. Those whose parents had fewer than 30 cigarettes a week had undetectable levels of the chemical.
Lead researcher Professor Stephen Hecht said: "The level of NNAL detected in the urine of these infants was higher than in most other studies."
The study, published in the journal Cancer Epidemiology, Biomarkers and Prevention, did not measure how smoke may affect children's development.