We report on a family with two sibs suffering from tuberous sclerosis. The parents were normal in all clinical tests including Wood's light examination of the skin, ophthalmoscopy, X-ray computerized tomography of brain, liver, and kidneys, cardiac echography and MR imaging of the brain. The most likely explanation is a germinal cell mosaic in one of the parents. A recurrence risk of 20 to 37% seems appropriate. The implications for risk assessment of sporadic cases are emphasized.