A prospective longitudinal study on the development of children of drug-dependent mothers who used combinations of cocaine, heroin and methadone during pregnancy was carried out in Amsterdam. The children and a contrast reference group were followed from birth to the age of 5 1/2 years. Cognitive functioning was assessed at 3 1/2, four, 4 1/2 and 5 1/2 years. The children of drug-dependent mothers repeatedly showed delays in cognitive functioning at preschool-age. Individual difficulties, as well as differences in developmental patterns over time, were found most clearly when facets of language development formed part of the assessments. Intervention programmes should focus on improvements of early language development and communication, in addition to the children's ability to adapt to task situations.