The perinatal mortality in Denmark decreased significantly from the late 1980's to 1990-1993 but increased among births by mothers aged 35-39 years, from 9.7 in 1985-1989 to 11.0 per 1000 born in 1990-1993. No increase was observed among infants of teen-age mothers or mothers older than 40 years. Based upon data from the Danish National Register of Causes of Death and the Birth Register all perinatal deaths in 1980-1993 were classified in nine categories including pathoanatomical, obstetrical and paediatric criteria. The increase in perinatal mortality in infants born to mothers aged 35-39 years was because of more deaths due to congenital birth defects, intra-partum events and foeto-placental dysfunction, while mortality due to prematurity decreased. The rate of multiple pregnancies and of primiparas increased significantly among 35-39 year-old mothers. With reference to international literature, the article discusses the possible impact of in-vitro fertilisation and other fertility treatments upon the special trend in perinatal mortality in this mother age-group. A national Danish IVF-register was first established in 1994 and will in the future allow concrete analyses of the outcome of IVF-pregnancies.