Of 35 patients with 48 angiomyolipomas 24 patients were followed clinically to determine the natural history of angiomyolipoma. Average patient age at presentation was 50 years (range 17 to 74) and of the patients 94% were women, 17% had tuberous sclerosis and 25% overall had bilateral disease. The patients could be divided into 2 distinct groups based on tumor size of 4 cm. or less and greater than 4 cm. Those with tumors less than 4 cm. were less likely to be symptomatic (24%) and patients with angiomyolipomas greater than 4 cm. were more often symptomatic (52%). No surgery was required for tumors less than 4 cm. but for 30% of the tumors greater than 4 cm. surgical intervention was necessary. Unlike any previously reported large series this study included radiological and historical followup available for 24 patients with angiomyolipoma with a mean followup time of 4 years (range 0.5 to 14). Moreover, to our knowledge we report for the first time documented growth during the study period of 27% of angiomyolipomas less than 4 cm. (4 of 15 tumors) and 46% of angiomyolipomas greater than 4 cm. (6 of 13 tumors). All patients with tumors less than 4 cm. were asymptomatic and only 1 required surgery. In contrast, tumors greater than 4 cm. were more frequently symptomatic (46%) and required surgery (54%). Patients with tuberous sclerosis and angiomyolipomas were distinctly different from patients with angiomyolipoma only, since they tended to present at a younger age, had a higher incidence of bilateral renal involvement, were more symptomatic, had larger tumors that were more likely to grow, and frequently required surgery. Based on this study, a modified approach to the current management of angiomyolipoma is recommended.