A 42-year-old baker had hyperkeratotic lesions on reticular erythema of his palms and volar digits. Attachment between the epidermis and the dermis was loose at the hyperkeratotic sites, but no blisters had formed. After he stopped work for 2 months, the skin lesions disappeared, leaving a slight scar. Immunofluorescence studies showed a normal distribution of type VII collagen and laminin, but there was almost no fluorescence for type IV collagen in the basement membrane zone. Disappearance of cytoplasmic organelles and aggregated tonofibrils was evident under electron microscopy in the basal cells which appeared shrunken under light microscopy. The structures of lamina lucida, basal lamina and anchoring fibrils were disarranged. The loose dermo-epidermal attachment is probably due in part to this disorganized basement membrane structure lacking type IV collagen, which may have been digested by heat-activated matrix metalloproteinases. In addition, his work-related chronic heat exposure might have caused hyperproliferation of keratinocytes.