Unenhanced abdominal CT scans of 35 patients with autosomal dominant polycystic kidney disease (ADPKD) showed multiple high-density (58-84 HU) renal cysts in 42.9% of patients, occasional high-density cysts in 25.7%, and no high-density cysts in 31.4%. These high-density cysts were usually subcapsular and were more frequent in patients with markedly enlarged kidneys and flank pain at the time of CT. Several were found to contain altered blood on pathological analysis. Follow-up CT often showed a reduction in cyst densities, although some cysts developed mural calcification and calcification of their contents. High-density cysts are probably produced by cyst hemorrhage. This may occur randomly as part of the natural history of the disease or may result from minor trauma to the enlarged kidneys. Renal carcinomas occur rarely in ADPKD and may occasionally be hyperdense. However, high-density cysts may usually be distinguished from carcinomas on CT by their smooth contours, sharp interfaces with renal parenchyma, homogeneity, and lack of contrast enhancement.