Mitomycin C is an alkylating chemotherapeutic agent which is instilled intravesically to prevent recurrence of superficial bladder carcinomas. After several cycles of mitomycin C, our patient developed a pruritic genital dermatitis and palmar desquamation. Following exclusion of a fungal infection, we performed patch tests using the standard series, the major basic ointment ingredients, disinfectants, and mitomycin C in concentrations of 0.001 to 0.1%; the resulting diagnosis was allergic contact dermatitis due to delayed-type hypersensitivity to mitomycin C. The skin rash rapidly resolved with application of topical steroids, and the intravesical chemotherapy was changed to doxorubicin. Eczematous skin reactions are quite common side effects after intravesical instillation of mitomycin C. In the majority of cases, they are caused by delayed-type hypersensitivity reactions, presumably elicited by hematogenous spread of the allergen, and not by irritation. The sensitization most likely occurs via the bladder mucosa. In order to differentiate between allergic and toxic contact dermatitis, patch tests with the above-mentioned mitomycin C concentrations are useful. In cases of mild allergic contact dermatitis the intravesical chemotherapy might be continued with concomitant topical steroids.