The development of two groups of children whose mothers had been given progesterone supplements during pregnancy to relieve symptoms of toxaemia was assessed, one group at 2 yr of age and the other at 16 yr of age. There was no evidence that progesterone supplements accelerated development in the 2-yr-old age group or enhanced intellectual and academic attainment in the 16-yr-old age group. In addition, the evidence regarding the reported beneficial effects on intellectual attainment of in utero exposure to excess sex steroids is discussed.