A 40-year-old female patient with diabetes mellitus presented with multiple erythematous ulcerated nodules and plaques predominantly on the trunk. A few months ago her partner had a small ulcer on the penis. She was HIV negative but showed markedly elevated syphilis serology titers (TPPA titer > 1:20.480, VDRL titer 1:128). The serum glucose levels exceeded 275 mg/dl. After exclusion of neurological involvement, we made the diagnosis of "lues maligna" arising in the setting of diabetes mellitus. The patient was treated with 2.4 million units benzathine penicillin intramuscularly weekly for three weeks. Simultaneously, diabetes therapy was improved with insulin injections. The syphilitic lesions cleared rapidly. In the follow-up VDRL titer was negative. "Lues maligna" is an unusual ulcerative variant of secondary syphilis which has been observed more frequently in HIV-infected patients in the last years. The occurrence of this aggressive variant in the clinical setting of diabetes mellitus is extremely rare.