OBJECTIVES: To study in detail the differences in infant mortality between Denmark and Sweden. STUDY DESIGN: Data retrieved from national health registers on infant death rates were compared for 1980-1988, supplemented with a study on the impact of socio-economic conditions in 1985-86 (Denmark) or 1986 (Sweden). RESULTS: Even after stratification for maternal age, parity, and socio-economic group, the Danish mortality rate was higher in all age-at-death intervals except for stillbirths. Maternal age-parity distribution was more favourable in Denmark, the socio-economic distribution in Sweden. The most marked country differences was seen in young women. The difference in the rate of perinatal deaths but not of later deaths is explainable by a more favourable birth weight distribution in Sweden than in Denmark. CONCLUSIONS: The studied variables do not explain the difference in mortality risk but it may be due to life style factors so far not identified.