Wilms tumor (nephroblastoma), a primary renal neoplasm containing primitive blastema and embryonic glomerulotubular structures, is seen rarely in adults. To identify clinical and radiologic criteria for preoperative diagnosis of adult Wilms tumor, we studied 29 cases reported in the literature from 1975 to 1987 (all patients were 15 years old or older) and four newly diagnosed cases. The mean age of patients was 30 years; 80% were less than 35 years old. Each patient presented with a large, rapidly growing, abdominal mass (average duration of symptoms, less than 2 months), had no constitutional symptoms (e.g., weight loss, fever), and otherwise were healthy (80%). Twenty-six patients had IV or retrograde pyelograms. Twenty-three showed a nonspecific mass effect. In seven (78%) of nine patients, abdominal CT scans showed a large, inhomogeneous mass with large areas of low density and increased enhancement of the compressed remaining normal renal parenchyma, which resembled a pseudocapsule. In five (63%) of eight patients, sonograms showed a complex mass with large cystic components. In 18 (82%) of 22 patients, arteriograms showed a hypovascular mass with fine wavy or zigzag (creeping-vine) neovascularity. We conclude that a rapidly growing renal mass in a young patient (less than 35 years old) that is shown to be complex and cystic by CT or sonography and that is hypovascular with fine, wavy neovascularity on arteriography is suggestive of adult Wilms tumor (75-80%). An awareness of this constellation of findings may be helpful in diagnosing this unusual tumor before surgery.