A case of mesoblastic nephroma, hypercalcemia, and raised levels of prostaglandins in a 2-month-old female infant is reported. Plasma parathyroid hormone (PTH) was normal and urinary prostaglandins were raised. During surgery a prostaglandin arteriovenous gradient was demonstrated. A large quantity of PGE was extracted from the tumor by radioimmunoassay after incubation. Both blood calcium and urinary prostaglandins returned to normal after nephrectomy. These results show that this tumor produced prostaglandins, the mediators of hypercalcemia in this patient.