A prospective follow-up study of 877 children born after ICSI was carried out. The aim of this study was to compile data on karyotypes, congenital malformations, growth parameters and developmental milestones so as to evaluate the safety of this new technique. The follow-up study included agreement to genetic counselling and prenatal diagnosis and was based on a physical examination at the Centre for Medical Genetics (Dutch-speaking Brussels Free University, Brussels, Belgium) at 2 months, 1 year and 2 years, when major and minor malformations and a psychomotor evolution were recorded. Between April 1991 and July 1995, 904 pregnancies obtained after intracytoplasmic sperm injection (ICSI) led to the birth of 877 children (465 singletons, 379 twins and 33 triplets). Prenatal diagnosis determined a total of 486 karyotypes, of which six were abnormal (1.2%) and six (1.2%) were familial structural aberrations, all transmitted from the father. This slight increase in de-novo chromosomal aberrations and the higher frequency of transmitted chromosomal aberrations are probably linked directly to the characteristics of the infertile men treated rather than to the ICSI procedure itself. In all, 23 (2.6%) major malformations were observed in the children born, defined as those causing functional impairment or requiring surgical correction. No particular malformation was disproportionately frequent. Compared with most registers of children born after assisted reproduction and with registers of malformation in the general population, the figure of 2.6% was within the expected range. These observations should be further completed by others and by collaborative efforts. In the meantime, patiens should be counselled about the available data before any treatment: the risk of transmitted chromosomal aberrations, the risk of de-novo, mainly sex chromosomal, aberrations and the risk of transmitting fertility problems to the offspring. Patients should also be reassured that there seems to be no higher incidence of congenital malformations in children born after ICSI.