The incidence of neuronal ceroid-lipofuscinoses (NCL) in West Germany was determined using a novel method which is applicable to other autosomal recessively inherited diseases. Questionnaires were sent to all pediatric departments (answer rate 189/276, 68%), schools for the blind (39/46, 85%), and neuropathological institutes (15/22, 68%). Diagnoses were accepted only when based on firm clinical and/or electron microscopic criteria; 207 such identified patients were sorted according to year of birth. Plotting the cumulative number of new cases per year against the year of birth resulted in a slightly S-shaped curve. Before the year 1962, the curve is relatively flat, probably due to inefficient case registration. Between 1968 and 1977, the slope of the curve is constant--a steep, nearly straight line. Thereafter the curve flattens out again, likely due to inefficient registration of young, still undiagnosed patients. We interpret the central segment of the curve, which is continuously straight over a period of 10 years and corresponds to 92 patients, as a period in which efficient registration of new cases occurred. The number of live births being 7,211,543 during the same period, the NCL incidence is calculated to be 1.28 per 100,000 live births (0.71 for juvenile NCL and 0.46 for late infantile NCL).