Objective: To report a case of renal failure associated with microsporidian infection in an HIV-seropositive patient. Design: Case report. Setting: Chelsea and Westminster Hospital, London, England, UK. Patient: An HIV-seropositive patient presented febrile with abdominal pain who developed renal failure. Penal biopsy and urinalysis showed infection with a microsporidian of the genus Encephalitozoon. Intervention: Treatment with albendazole (400 mg) twice daily was associated with disappearance of infection from the urine, clinical improvement and return of renal function virtually to normal. Conclusion: HIV-seropositive individuals with renal failure should have urine screened for microsporidia. The administration of albendazole in such cases may reverse renal failure.