An epidemiological study of 439 cases (184 males, 255 females) of pheochromocytoma (paraganglioma) diagnosed in Sweden from 1958-81 is presented. From the mid 1960's an average of 22 cases have been diagnosed each year. In 255 cases, the pheochromocytoma was diagnosed by clinical examination and/or surgery. In 184 cases (40%), the diagnosis was achieved by autopsy and out of which 60 (14% of all cases) were reported as incidental findings. Ninety-five pheochromocytomas (22%) were located outside the adrenals. The average (+/- SD) age at diagnosis for the total study population was 55.8 +/- 17.7 years. Clinically diagnosed tumour cases were significantly younger (48.5 +/- 16.3 years) compared to cases diagnosed at autopsy (65.8 +/- 14.0). Pheochromocytomas were more common among women than among men. The age specific incidence rate increased continuously for both sexes from the youngest to the oldest age groups. The average incidence of pheochromocytoma in the 24 counties of Sweden was 2.1 cases per million inhabitants per year, range 0.9-5.3. A North to South gradient was noted with a higher incidence in the Southern parts of the country. Whether this is due to a real geographical variation of incidence, to a larger number of unreported cases in the Northern parts, or to presence of families with Sipple's syndrome in the South is unclear.