"Trans-placental" exposure to polycyclic aromatic hydrocarbons (PAHs) in newborn babies of mothers smoking during pregnancy was investigated and compared with that in newborn babies of non smoking mothers. PAHs intake was evaluated by measuring urinary levels of 1-hydroxypyrene, a pyrene metabolita and a biological index of exposure. 42 mothers-babies couples (31 smoking and 11 non smoking mothers) were examined. Urinary concentrations of 1-hydroxypyrene in 42 controls, 18 non smoking and 24 smoking women, were determined at the same time. Mean values of 1-hydroxypyrene in non smoking women in labour and their babies were 0.15 (SD 0.11) and 0.15 (SD 0.10) micrograms/g creatinine respectively. In controls the mean was 0.15 (SD 0.09) micrograms/g creat. Mean values of 0.23 (SD 0.17) and 0.20 (SD 0.15) micrograms/g creatinine were determined in smoking mothers and their newborn babies, whereas in smoking controls 1-hydroxypyrene was 0.42 (SD 0.45) micrograms/g creatinine. The highest levels of 1-hydroxypyrene were detected in smokers (both controls and women in labour). Babies of smoking mothers also showed a higher mean value of 1-hydroxypyrene than babies of non smokers. However the only statistically significant difference found was between smoking and non smoking controls.