Normal growth and development, as well as the prevention of overweight, are major goals in the treatment of paediatric patients with insulin-dependent diabetes mellitus (IDDM). We therefore evaluated longitudinally the anthropometric measurements of height and weight, as well as bone age, in an unselected group of 389 patients with IDDM treated at one institution. In order to identify genetic influences on these parameters, height and weight were determined in 186 unaffected siblings and 177 pairs of parents. At diagnosis, patients were slightly taller than average (median z score: +0.37). During the subsequent course of diabetes, age-adjusted heights decreased progressively for the first 9 years, catching up again after more than 10 years of diabetes. Bone ages were progressively retarded with increasing duration of diabetes. In 76 patients of 18 years or older, median z-score for height was +0.30, not different from their unaffected siblings (median z-score: +0.22). The correlation with midparental height was identical for diabetic and nondiabetic siblings (r = 0.43). In contrast, children with diabetes were significantly heavier (z-score for weight: +0.74 compared to +0.34 in unaffected siblings; p < 0.002). Obesity developed primarily during and after puberty. We conclude that: 1) during the course of diabetes, longitudinal growth is temporarily reduced and maturation is delayed in children with diabetes compared to unaffected siblings. However, this effect of diabetes is transient and small compared to genetic influences on height in an individual child. 2) As a group, children with IDDM become significantly overweight, which is likely to increase the cardiovascular risk during adulthood.