Prenatal diagnosis of trisomy 18 by amniocentesis in the latter half of pregnancy is now a common event. Accurate prognostic information is crucial for families making decisions about delivery management. Three recently published studies showed much shorter survival for trisomy 18 than was reported by earlier papers. For this reason, we studied trisomy 18 survival. We examined chromosome laboratory records to find all trisomy 18 diagnoses made in Utah between 1979 and 1988. Death certificates and hospital records were used to determine survival. We found 64 live-born cases with trisomy 18 out of 388,563 total births over the 10-year period, a prevalence of 1/6071. Our results show a median survival of 4 days and a 1 week survival of 45%, similar to that reported in the 3 recent studies. However, we had a significantly greater survival at 6 months (9% in Utah versus 3% in Denmark) and 1 year (5% versus 0 in the 3 studies). In contrast to recent studies, earlier investigations showed 80% survival at 2 weeks and 8% at 1 year. It is not surprising that recent studies show shorter survival, since in the 1960s the diagnosis was typically not made until age 2 months. With prenatal and neonatal diagnosis many cases which would have died prior to detection in earlier times are now diagnosed. The longer survival discrepancies are more difficult to explain, but may simply be due to small numbers. © 1994 Wiley-Liss, Inc.