A 70-year-old white man entered hospital complaining of low-grade, daily fever. weight loss, and weakness of 3 months duration. Positive findings were 14,000 leukocytes per mm3 with 3% eosinophils, and interstitial infiltration of the right midlung. Readmitted 13 weeks later, because of persistent fever, weakness, and cough, he had 15,150 leukocytes per mm3 with 5% eosinophils, 32% hematocrit, and radiographic evidence of cavitation in the area of infiltration in the right midlung. One month later, eggs and rhabditoid larvae of Strongyloides stercoralis were found in blood-streaked sputum. Thiabendazole was given at a dosage of 8 mg per pound of body weight, twice daily, for 3 weeks. Live rhabditoid larvae and eggs of S. stercoralis continued to be excreted in blood-streaked sputum, and filariform larvae were obtained in cultures of sputum. A stool specimen obtained at the end of thiabendazole therapy contained rhabditoid larvae. The patient died from massive hematemesis, 3 days after completion of therapy; an autopsy was not done. The patient received corticosteroid therapy for 1 month before the first sputum specimen was examined, until death, because he was believed to have carcinomatosis. We believe that this therapy may have induced autoinfection and may have inhibited the effectiveness of thiabendazole.