The malignant degeneration of a skin ulcer has been reported as early as in 1884. In spite of this, however, the danger of malignant degeneration of sinus tracts associated with recurrences of chronic osteomyelitis is still underestimated. This development is neither restricted to case histories of years and decenniums nor to elderly patients and may also involve any part of the skeleton. Usually patients become aware of suddenly increasing tenderness and purulation, "washed out" structures of osteolytic areas are demonstrable radiographically. Diagnosis is established by mandatory histological examination of sinus tracts. Amputation or exarticulation of the extremity involved including efferent lymphatic vessels and nodi has no alternative. Local measures are insufficient. Apparently the first 3 years are decisive as to the prognosis. The prophylaxis consists of the therapy of the original disease: radical debridement of the osteomyelitic focus and filling of the defect with autogenous cancellous bone grafts.