We identified more than 30 studies on the association between exposure to 
maternal tobacco smoke during pregnancy and cancer in childhood. We combined 
their results in meta-analyses based on a random effects model. The results of 
the meta-analyses suggest a small increase in risk of all neoplasms [relative 
risk (RR) 1.10; 95% confidence interval (CI), 1.03-1.19; based on 12 studies], 
but not of specific neoplasms such as leukemia (RR 1.05; CI, 0.82-1.34; 8 
studies) and central nervous system tumors (RR 1.04; CI, 0.92-1. 18; 12 
studies). Results for other specific neoplasms were sparse, but the available 
data did not suggest a strong association for any type of tumor. No clear 
evidence of dose response was present in the studies that addressed this issue. 
The results on exposure to maternal tobacco smoke before or after pregnancy are 
too sparse to allow a conclusion. The results on exposure to paternal tobacco 
smoke suggest an association with brain tumors (RR 1.22; CI, 1.05-1. 40; based 
on 10 studies) and lymphomas (RR 2.08; CI, 1.08-3.98; 4 studies). The data are 
too sparse for the other neoplasms, although the results of a few recent large 
studies are compatible with a weak carcinogenic effect of paternal smoke. For 
exposure from either maternal or paternal smoke, bias and confounding cannot yet 
be ruled out. Further studies are needed to confirm the hypothesis that parental 
tobacco smoke, from the father in particular, is a risk factor of childhood 
cancer. Results on the risk of lung cancer in adulthood and childhood passive 
smoking exposure are available from 11 studies: they do not provide evidence of 
an increased risk (summary RR 0.91; CI, 0.80-1.05).