Body weight regulation after treatment was studied in 87 hyperthyroid and 18 hypothyroid subjects. Mean body weight was 83.9 percent of the premorbid weight at the time of initial treatment for hyperthyroidism and 102.5 percent at 96 months following treatment. Mean (+/- SD) post-treatment weight difference from baseline was 3.4 +/- 18.6 pounds at 96 months (n = 44). Early weight gain was greatest in subjects in whom thyroxine values normalized quickly. Hyperthyroid subjects from whom a body mass index could be calculated (n = 45) were divided into two groups. The obese group had a greater mean weight loss (35.2 +/- 15.0 pounds versus 21.2 +/- 9.9 pounds, p less than 0.001) and a lower weight (percent of baseline weight) at the time of treatment for hyperthyroidism (81.6 +/- 7.7 percent versus 86.0 +/- 5.1 percent, p less than 0.05). Subjects with thyroxine levels of 20 micrograms/dl or more had higher premorbid body weights and greater weight loss from baseline than subjects with thyroxine values below 20 micrograms/dl. Hypothyroid subjects showed a small decline in mean body weight over the first six months of treatment but returned to pretreatment weight by 24 months. In the absence of significant metabolic derangement, body weight is regulated within narrow limits over many years. Effective treatment of hyperthyroidism is accompanied by weight gain.