The authors analyzed the prevalence of obesity by family size and birth order for a total population of over 280,000 19-year-old Dutch males, who were born between 1944 and 1947 and who were from one- to five-child families. Obesity was defined by a relative weight index, 120% or more of the standard of weight for height. Overall obesity prevalence was 1.83%; sons of manual workers were more prone to obesity than sons of non-manual workers. Individuals from one-child families (only children) were uniquely at risk for obesity, particularly in the non-manual social class. This result was similar to reported findings for childhood obesity.