Large differences in infant mortality are reported among and within industrialised countries. We hypothesised that these differences are at least partly the result of intercountry differences in registration of infants near the borderline of viability (<750 g birthweight) and/or their classification as stillbirths vs. live births. We used the database of the International Collaborative Effort (ICE) on Perinatal and Infant Mortality to compare infant mortality rates and registration practices in Norway (n = 112484), Sweden (n = 215 908), Israeli Jews (n = 148123), Israeli non-Jews (n = 52 606), US Whites (n = 6 074 222) and US Blacks (n = 1328332). To avoid confounding by strong secular trends in these outcomes, we restricted our analysis to 1987-88, the most recent years for which data are available in the ICE database for all six groups. Compared with Norway (with an infant mortality rate of 8.5 per 1000), the crude relative risks [95% confidence intervals] were 0.75 [0.69,0.81] in Sweden, 0.97 [0.90,1.06] in Israeli Jews, 1.98 [1.81,2.17] in Israeli non-Jews, 0.95 [0.89,1.01] in US Whites and 2.05 [1.95,2.19] in US Blacks. For borderline-viable infants, fetal deaths varied twofold as a proportion of perinatal deaths, with Norway reporting the highest (83.9% for births <500 g and 61.8% for births 500-749 g) and US Blacks the lowest (40.3% and 37.6% respectively) proportions. Reported proportions of live births <500 g varied 50-fold from 0.6 and 0.7 per 10000 in Sweden and Israeli Jews and non-Jews to 9.1 and 33.8 per 10000 in US Whites and Blacks respectively. Reported proportions 500-749 g varied sevenfold from 7.5 per 10000 in Sweden to 16.2 and 55.4 in US Whites and Blacks respectively. After eliminating births <750 g, the relative risks (again with Norway as the reference) of infant mortality changed drastically for US Whites and Blacks: 0.82 [0.76,0.87] and 1.42 [1.33,1.53] respectively. The huge disparities in the ratio of fetal to infant deaths <750 g and in the proportion of live births <750 g among these developed countries probably result from differences in birth and death registration practices. International comparisons and rankings of infant mortality should be interpreted with caution.