Rates and survival figures for trisomy 13 and trisomy 18 have been calculated for Denmark (DK) based on a 10-year period (1977-86). The data have been ascertained through The Danish Central Cytogenetic Register, all cytogenetic laboratories in DK, paediatric departments throughout the country. The Medical Birth Register and The Register of Causes of Death in DK. Nineteen liveborn probands with trisomy 13 and 76 liveborn probands with trisomy 18 were found. No stillborn cases with trisomy 13 but 6 with trisomy 18 were found. By prenatal diagnosis 19 probands with trisomy 13 and 46 with trisomy 18 were found. Based on liveborn and stillborn probands, the prevalence at birth was 1 per 29,374 for trisomy 13 and 1 per 6806 for trisomy 18. The median survival for trisomy 13 was 2.5 days, while the same figure for trisomy 18 was 6.0 days. The rates calculated seem rather low compared to earlier studies.