Oropharyngeal candidiasis is frequently a complication of patients with altered immune states. Clotrimazole troches are effective in the treatment of Candida and were evaluated in this study in a prophylaxis regimen. Patients with malignant neoplasms who were receiving chemotherapy and renal transplant recipients who were receiving immunosuppressives were randomized to receive either clotrimazole (10 mg) or placebo troches three times a day in a prospective, double-blinded study. Eighty-four patients were randomized into the study, 18 patients with leukemia, 19 patients with malignant neoplasms, and 47 patients with renal transplants. Among all patients, thrush developed in 57% while receiving placebo compared with 13% while receiving clotrimazole prophylaxis. Prophylaxis showed significant benefit for the renal transplant recipients and for patients with solid malignant neoplasms, but not for the leukemic patients. Clotrimazole troches are effective in preventing oral candidiasis in a select group of patients.