The aim of the study was to analyse the effects on mortality of body height and body mass index (BMI) in young men, taking into account social and behavioural characteristics in early youth. The study is a 20-year follow-up of all Swedish men (n = 50,465) conscripted for military service in 1969-70. Baseline data on body height and weight as well as interview and questionnaire data on social and behavioural characteristics were linked to mortality data up to the end of 1988. Nine hundred and forty-six men died during the follow-up. We found a significant increase in mortality by decreasing body height; persons below 165 cm had an almost twofold increased mortality compared with those of average height. This association was reduced to the limit of statistical significance when we controlled for social and behavioural characteristics in a multivariate model. Persons with a BMI above 30 had a more than twofold increased mortality compared with those of average BMI. This association remained highly significant in a multivariate model. We thus confirmed previous findings of an inverse association between body height and mortality, but found that this association was almost entirely due to social background factors. The independent association between BMI and mortality may indicate a stronger genetic influence on BMI than on body height.