The children of mothers who smoked during pregnancy (n = 1819) and their controls, all born in northern Finland in 1966, were followed up to the age of 14. The children of the smokers were more prone to respiratory diseases than the others. They were also shorter and their mean ability at school poorer than among the controls. The differences remained significant after adjusting for the mother's height and age, social class as determined by the father's occupation, number of older and younger children in the family and the sex of the child. The smoking mothers were found to differ from their controls in each social class, in contrast to the situation at the time of pregnancy, now having on average poorer health, being more often unemployed and having more often left their families. Maternal smoking was still seen to have an effect on the children's physical and mental development, even when these factors were also taken into consideration in the regression analyses, although it was less important than many other socio-biological factors, and it was no more important than paternal smoking.