The weights of 762 normal thyroid glands dissected at autopsies in New York City indicate that the mean adult thyroid weighs 16.7 ± 6.9 g. The thyroid weights are distributed log-normally. These mean values are lower than the values described by other workers or used in standard dose measurements. The data suggest that 1 per cent of New York residents have thyroids that weigh about one-third of the mean. Data are also presented on a series of fetal thyroids in which the weights varied from 20 mg at the third month of gestation to 1.5 ± 0.6 g at full term.