The development of adiposity was followed in 164 subjects from the age of one month to adulthood. The 25th and 75th centiles of the weight/height2 (W/H2) index were chosen as cut-off points to define the lean, medium and fat subjects at both one and 21 years of age. Only 42% of the children remained in their original category, that is 41% of the lean infants at one year stayed lean, 42% of the medium infants stayed medium and 41% of the fat infants stayed fat. Accordingly, most fat infants did not stay fat, but twice as many fat as non-fat infants became fat adults (41 and 20% respectively). The relative risk of being fat adults was 1 for the lean, 1 for the medium and 2 for the fat infants at one year. Several paths of development emerged: they were related to age at the second rise in adiposity, termed adiposity rebound, which usually occurs at about six years, as observed on skinfold thickness and W/H2 charts. The earlier the rebound, the higher the adiposity at adult age, whether this was measured by W/H2 index or subscapular skinfold. The cohorts of children who left the channel they had been following included fat infants with a late rebound who subsequently returned to normal, and lean infants with an early rebound who grew fatter. Other cohorts remained in their original groups, for example, fat infants with an early rebound who stayed fat and lean infants with a late one who stayed lean. Age at rebound provided two indications: the existence of a regulartory process among the transiently fat or lean infants who returned to average after a late or early rebound respectively, and pathological development among the children who became fat or lean after an early or late rebound. Age at rebound is an indicator of the subsequent development of fatness.