Topical hypersensitivity to corticosteroids was studied by epicutaneous testing using the Finn Chamber technic. The steroids were tested in both ethanol and white petrolatum and, in certain cases, in dimethyl sulfoxide. Additionally, commercial preparations were tested. Three groups of patients were studied: (1) patients with a history of hypersensitivity to at least two topical preparations (five of ten patients studied showed a positive patch test reaction for corticosteroids), (2) patients in whom topical corticosteroid hypersensitivity was suspected because of treatment-resistant eczema (seven of twenty-five patients showed a positive patch test reaction), and (3) dermatologic inpatients and outpatients undergoing epicutaneous testing for suspected topical hypersensitivity. Hydrocortisone-17-butyrate (H-17-B) was included in the standard patch test series; of 450 patients tested, two showed a positive patch test reaction. All the patients with corticosteroid hypersensitivity had a positive reaction to H-17-B. In six patients, additional hypersensitivities to one or several other steroid preparations were seen. Use testing was performed as an open test, with 0.1% or 1% H-17-B in ethanol on normal skin of the flexor side of the upper extremities. A positive test reaction was seen in only one of nine patients. Results of use testing with the commercial 0.1% H-17-B (Locoid) ointment were always negative. Our study suggests that the sensitivity of patch tests for corticosteroid hypersensitivity can be increased by using ethanol as vehicle.