All human obesity is characterised by adipocyte hypertrophy and when body weight exceeds 170 per cent of ideal, a maximum cell size of roughly twice normal is achieved. With greater severity, hyperplasia becomes increasingly manifest and when body weight exceeds 170 per cent of ideal, the degree of hyperplasia is well correlated with severity. Although severity and hypercellularity are often found in those with early onset of obesity, individuals can be found who are hypercellular but have had a later onset of obesity. Until the details of cellular development in man are more fully understood, the precise timing of 'critical' periods for cellular development must remain speculative.