The manifestations, clinical course and treatment of 14 patients with non-malignant renal vein thrombosis are described. Most patients (10 of 14) had generalized vague illness and nephrotic syndrome but 4 were initially seen with acute symptoms of flank pain, hematuria or hypertension. Renal vein thrombosis affected young men 2.5 times more often than women and occurred on the left side 2.6 times more commonly than on the right or both sides. Red blood cell casts in the urinary sediment, heavy proteinuria and hypoalbuminemia were useful indicators of the disease. Excretory urographic signs were suggestive of renal vein thrombosis in all patients and these were corroborated by angiographic studies. Systemic anticoagulation with or without a renal failure program and diuretics, or simply a combination of the last 2 modalities, was used in 9 patients. In 2 of the 9 patients who were unresponsive the adjuvant use of cyclophosphamide and steroids effected a cure. The remaining 5 patients underwent nephrectomy or thrombectomy. All 14 patients were followed for 1 to 7 years (mean 1.6 years). Ten patients were cured or improved, 1 patient was unchanged, and in the remaining 3 patients the condition deteriorated and they subsequently required a renal allograft. The rationale for various forms of treatment is discussed.