Adoptive immunotherapy with recombinant interleukin-2 has been complicated by significant nephrotoxicity of uncertain etiology. Creatinine clearance, effective renal plasma flow, plasma renin activity, aldosterone levels, and urinary prostaglandin excretion were evaluated in a 62-year-old man receiving continuous infusion recombinant interleukin-2. There was a marked decrease in the creatinine clearance and renal plasma flow, accompanied by an elevation in plasma renin activity and aldosterone level. Prostaglandin excretion also decreased, implying a direct effect on renal prostaglandin synthesis. The decrease in renal prostaglandin synthesis at a time of increased plasma renin activity may explain the reduction in renal function seen with recombinant interleukin-2 therapy.