We used data from a Scandinavian prospective multicenter study to investigate if smoking in pregnancy may have an adverse effect on the child's mental and motor abilities. Eligible for enrolment were para I and 2 women with a singleton pregnancy, who resided in one of the study areas and could be registered before the 20th gestational week. Women were classified as 'smokers' or 'non-smokers' at study start. At 13 months, 376 children (124 children of smokers) were evaluated with the Bayley Scales of Infant Development. At this age, children of smokers and non-smokers performed equally well. At 5 years, 369 children (132 children of smokers) were tested with the Wechsler Preschool and Primary Scales of Intelligence Revised (WPPSI-R), and 362 children with the Peabody Developmental Motor Scales (PDMS). Children of smokers had an increased risk of getting a WPPSI-R score below the median value of the population (OR = 2.1, 95% CI: 1.2-3.3), but the risk was reduced when we adjusted for maternal education (OR = 1.6, 95% CI: 0.9-3.7). Children of smokers had an increased risk of getting a test score below the median population value on the subscale 'balance' from PDMS (OR = 1.8, 95% CI: 1.2-2.8). Thus, we found that smoking in pregnancy was associated with a small, but demonstrable adverse effect on the child's balance at 5 years, whereas the negative effect on cognitive function did not reach statistical significance, when we adjusted for the mother's level of education.