Between 1961 and 1979, 349 patients with autonomously functioning thyroid nodules (AFTN) were seen. There were 287 nontoxic and 62 toxic lesions. Toxic lesions were seen in 56.5% of AFTN patients over 60 yr of age but in only 12.5% of the younger patients. The female to male ratio was 14.9:1 for nontoxic AFTN patients but only 5.9:1 for toxic AFTN patients. The proportion of toxic AFTN in patients less than 20 yr old (13.8%) was not significantly different from that in patients 20--60 yr old (12.7%). T3 toxicosis was observed in 46% of those patients with hyperthyroidism. All but 4 of the toxic AFTN measured 3 cm in diameter or larger. AFTN 3 cm or larger were more than twice as common in patients 40 yr or older than in younger patients. Of 159 untreated nontoxic AFTN patients, 14 became toxic within 1--6 yr. One patient in 5 with a nontoxic AFTN 3 cm or larger developed toxicity.