To determine the incidence, size, and outcome of spontaneous massive fetomaternal transplacental hemorrhage in Manitoba. The Kleihauer maternal-fetal screening records at the Rh Laboratory were reviewed for the period October 1970 to December 1992. Rh Laboratory correspondence for the 7-year period ending December 31, 1992, was reviewed to determine those instances of fetomaternal transplacental hemorrhage for which there was knowledge regarding fetal and neonatal outcome. Twenty-seven of 30,944 Rh-negative women undergoing routine Kleihauer screening at delivery had fetomaternal transplacental hemorrhages of at least 80 mL of fetal blood (incidence one in 1146 pregnancies); in 11 of the 27, the hemorrhages were 150 mL of blood or more (one in 2813 pregnancies). In non-routinely screened, selected maternal blood samples, sent because of unexplained fetal distress, fetal death, or neonatal anemia, 36 had evidence of fetomaternal transplacental hemorrhage of at least 80 mL of blood, 28 of which were in the 7-year period ending December 31, 1992 (one in 3893 pregnancies). Of these 28, 25 had hemorrhages of at least 150 mL of blood (one in 4360 pregnancies). Ascertainment of fetomaternal transplacental hemorrhage of 150 mL or more in women referred because of fetal or neonatal problems was 2813/4360 x 100 = 64.5%. Twenty-six cases in the final 7-year period had information regarding perinatal outcome. The incidence of adverse outcomes following massive fetomaternal transplacental hemorrhage was 46% (12 of 26). There were ten perinatal deaths, one infant death at 6 months, and one infant with spastic quadriplegia. Massive fetomaternal transplacental hemorrhage is uncommon but not rare, and subsequent adverse outcomes are common. A high index of suspicion with prompt investigation and appropriate management may improve the perinatal outcome following massive fetomaternal transplacental hemorrhage.