We report a patient with AIDS and intestinal microsporidiosis. While undergoing treatment with albendazole, he developed worsening diarrhea with abdominal pain and fever. The diagnosis of pseudomembranous colitis was made by flexible sigmoidoscopy and a positive stool specimen for Clostridium difficile toxin. The patient's symptoms resolved with oral vancomycin and his stool C. difficile toxin became negative. Albendazole is an antibiotic that is chemically related to metronidazole. Although a few case reports link metronidazole with the development of pseudomembranous colitis, albendazole has not been associated with the development of this condition. The spectrum of antimicrobial activity of albendazole and its efficacy in the treatment of intestinal microsporidiosis are reviewed. Pathogenic mechanisms for the development of pseudomembranous colitis and the epidemiology of this condition in patients with AIDS are discussed.