Abstract A large experiment was initiated in Denmark in 1972 to examine the benefits deriving from systematic crossing among dairy cattle breeds of similar size and with high protein and butterfat yields. In the first phase purebred lines of Red Danish (28) and Finnish Ayrshire (28) females were compared with a line of Danish Friesian females (28) which were mated with Holstein-Friesian bulls, and to two lines of Red Danish females (each of 240) that were mated with either Finnish Ayrshire or Holstein-Friesian bulls. Foundation females were sampled at random from purebred populations in the respective countries, while sires were selected from the best available. The foundation females produced a total of 1510 progeny over an eight year period. Least-squares analysis of variance were conducted to provide estimates of breed effects for production and reproduction traits of foundation females and for birth traits of their calves. Finnish Ayrshire cows were significantly lighter (587±14 kg) than Red Danish (642±5) or Danish Friesian (659±14) cows at maturity. Finnish Ayrshire cows produced significantly more butterfat in first lactation than Red Danish cows. Yields of butterfat (305-day) during first lactation were 212±7, 193±3 and 195±7 (kg) for Finnish Ayrshire, Red Danish and Danish Friesian, respectively. There were no significant differences among breeds for lifetime production, but due to better longevity Finnish Ayrshire cows were non-pregnant (empty) for a significantly longer period during their lifetime than Red Danish cows (442±31 and 374±12 days, respectively). Red Danish females received significantly more veterinary treatment for nutritional diseases, 1.2±0.1, than either Finnish Ayrshire, 0.7±0.2, or Danish Friesian, 0.8±0.2, and Finnish Ayrshire cows received significantly less treatments for feet problems 0.5±0.2, than either Red Danish, 1.1±0.1, or Danish Friesian, 1.3±0.2, over their lifetime. Finnish Ayrshire calves were significantly lighter at birth (38.9±0.6 kg) than either Red Danish (41.2±0.5) or Friesian (41.0±0.6) calves (Holstein-Friesian x Danish Friesian) even though Red Danish calves had a significantly longer gestation length (280.6±0.3 days) than Finnish Ayrshire (279.6±0.5) and Friesian (278.9±0.5) calves. There were no significant differences in calf mortality among these three groups, but Finnish Ayrshire cows had significantly less calving difficulty at first calving than the other breeds. Red Danish dams had a longer empty period following calving than Friesian dams. Holstein-Friesian x Red Danish crossbred calves were the heaviest of all groups (42.2±0.3 kg), while Finnish Ayrshire × Red Danish had the highest level of calf mortalities (8±1 %). Red Danish cows giving birth to crossbred calves had an empty period 8 days shorter (p<0.05) than those giving birth to purebred calves. Otherwise, there was little effect of crossbreeding on subsequent reproduction of dams. When adjustment was made for underlying traits, breed of calf effects generally diminished. In the case of calving difficulty adjustment for birth weight removed significant breed differences. When calf mortality was adjusted within breed groups for birth weight, gestation length and calving difficulty, Finnish Ayrshire and Finnish Ayrshire × Red Danish calves had higher mortality than the other breed groups. The effects of these underlying variables are reported and discussed, particularly in relation to incidence of calf mortality and retained placentas.