OBJECTIVE To report a case of trimethoprim/sulfamethoxazole (TMP/SMX)-induced hypoglycemia in an immunosuppressed renal transplant patient. METHODS English-language journal articles and reference texts identified via a MEDLINE search and a bibliographic review of pertinent data sources. RESULTS Hypoglycemia resulting from the combination of sulfonylureas and sulfonamides is a recognized drug interaction. Hypoglycemia induced by sulfonamides alone may be encountered less frequently. Previously reported cases of TMP/SMX-induced hypoglycemia postulated that the sulfonamide mimics hypoglycemic sulfonylurea agents and stimulates pancreatic islet cells to secrete insulin. We report a case of hypoglycemia following the administration of high-dose TMP/SMX in a renal transplant patient. Elevated C-peptide concentrations following the hypoglycemic episode indicate that hypoglycemia resulted from increased endogenous insulin secretion. CONCLUSIONS Hypoglycemia has been a rarely encountered result of TMP/SMX use. Patients receiving TMP/SMX, particularly those with impaired renal function and those receiving high doses, should be monitored closely for hypoglycemia.