504 overweight children admitted to hospital between 1921 and 1947 were followed up for 40 years by questionnaires at 10 year intervals. The mean weight for height (W/H) standard deviation score (SDS) reached a maximum in puberty (+3.5). The SDS fell to about +1 in adulthood. 47% patients were still obese (SDS greater than +1) in adulthood; 84.6% of these had SDS more than +2 in childhood. The degree of obesity in the family (parents and grandparents) and the degree of overweight in puberty were the most important factors for weight level in adulthood. Even when their food intake was in accordance with recommended levels, obese children had higher than normal weight as adults. Excessive overweight in puberty (SDS greater than +3) was associated with higher than expected morbidity and mortality in adult life. Weight-reducing measures should be started early in life to improve the unfavourable long-term prognosis for very obese children.