TSC in the kidney is expressed principally as renal cysts and angiomyolipomas. Both abnormalities may occur separately or together, and both are commonly multiple and bilateral. Cystic disease is sometimes so severe as to be confused with polycystic kidney disease, although the histopathologic findings are practically diagnostic of TSC. Severe cystic disease causes renal insufficiency; large angiomyolipomas predispose to life-threatening hemorrhage. Renal malignancies have been reported in what appears to be a significant number of patients. We hypothesize that the renal abnormalities result from cell hyperplasia and hypertrophy, much like the other abnormalities of tuberous sclerosis, such as cerebral tubers and cardiac rhabdomyomas. The renal abnormalities can therefore be regarded as an expression of the TSC gene, and their recognition as such carries importance for treatment and counseling.