Background. Intracytoplasmic sperm injection (ICSI) was introduced as a new form of in-vitro fertilisation (IVF) in 1993 and is now accepted as the treatment of choice for severe male infertility in many centres around the world. However, there is little information about the long-term outcome of children conceived by ICSI. We aimed to find out the medical and developmental outcome of children conceived by ICSI at age 1 year. Methods. In this prospective study, we compared the medical and developmental outcome at 1 year of 89 children conceived by ICSI with 84 children conceived by routine IVF, and with 80 children conceived naturally. Formal developmental assessment was done with Bayley Scales of Infant Development (2nd edition) from which a mental development index (MDI) was derived. Findings. There was no significant difference in the incidence of major congenital malformations or major health problems in the first year of life. However, the mean Bayley MDI was significantly lower for the children conceived by ICSI than for the children conceived by routine IVF or naturally (95.9 [SD 10.7], 101.8 [8.5], and 102.5 [7.6], respectively, p < 0.0001). 15 (17%) of 89 children conceived by ICSI experienced mildly or significantly delayed development (MDI < 85) at 1 year compared with two (2%) of the 84 children conceived by IVF and one (1%) of the 80 children conceived by natural conception (p < 0.0001). Interpretation. Although most children conceived by ICSI are healthy and develop normally, there is an increased risk of mild delays in development at 1 year when compared with children conceived by routine IVF or conceived naturally. These findings support the need for ongoing developmental follow-up of children conceived by ICSI to see whether they are at increased risk of intellectual impairment or learning difficulties at school age.