The effect of maternal smoking during pregnancy on the morbidity and mortality of the child up to the age of five was studied in 12068 births. The children of the smokers were compared with those of controls of similar age, parity, marital status and place of residence. Perinatal mortality was no higher among the smokers, but postneonatal mortality from 28 days to 5 years was almost significantly (p less than 0.05) higher. The children of the smokers were highly significantly (p less than 0.001) more often hospitalized in pediatric departments, the difference being clearest below the age of one. The average duration of hospital admissions was longer among the children of the smokers, and similarly the numbers of visits to the doctor and hospital admissions to any hospital under the age of one were more frequent among the children of the smokers. Respiratory diseases caused highly significantly more hospitalizations among these children.