Twelve female patients with necrobiosis lipoidica diabeticorum (six with diabetes and six without) had a 5-mm punch biopsy of the skin lesion performed. The tissue was processed for dermatopathologic examination in 12 cases and for direct immunofluorescence in 11. Vasculopathy with inflammation and thickening of vessel walls, at times leading to occlusion, was found in lesional skin in all 12 cases. Vessels contained deposits of immunoreactants in the involved skin in 11 cases. This included IgM in six, C3 in nine, fibrin in ten, IgG in one, and IgA in two. Vessels contained deposits of immunoreactants in uninvolved skin in seven patients (C3 in four, IgM in three, fibrin in three, C4 in one, and IgA in one), three of whom had type I diabetes.