The frequency and importance of complications of IVF and other ovulation induction (OI) are poorly known. We examined the occurrence of serious complications and miscarriages leading to hospitalization or operation after IVF (including microinjections and frozen embryo transfers) and OI treatment (with or without insemination). Women who received IVF (n = 9175) or OI treatment (n = 10 254) 1996-1998 in Finland were followed by a register linkage study until 2000. After the first IVF treatment cycle, 14 per 1000 women had a serious case of OHSS (ovarian hyperstimulation syndrome), with 23 per 1000 throughout the study period (mean of 3.3 treatments). The corresponding values after OI were very low. The rates of registered ectopic pregnancies and miscarriages after IVF were nine and 42 respectively per 1000 women, with corresponding rates after OI of eight and 42. Infections and bleeding were not common after IVF and even rarer after OI. Overall, 15% of IVF and 8% of OI women had at least one hospital episode during the study period. Though there was a low risk of complications after each IVF treatment cycle, repeated attempts resulted in serious complications for many women, and these occurred much more often than after ovulation induction alone.