SummaryA patient aged 16 years with a new mechanobullous disease is described. He had suffered from trauma‐induced bullae since birth. The lesions increased in hot weather and healed without scarring. The clinical picture was suggestive of Weber‐Cockayne or a mild Köbner type of epidermolysis bullosa simplex. Ultrastructural investigation of afresh blister revealed intraepidermal blister formation via cytolysis of basal cell cytoplasm at or above the hemidesmosomes to which were attached clumps of tonofilaments. On the floor of the blister the basal lamina and anchoring fibrils were normal. This type of blister morphology is characteristic of epidermolysis bullosa herpetiformis of Dowling‐Meara so that this patient has either a mild atypical form of the above disease or his case represents an example of a new subgroup of epidermolysis bullosa produced by a spontaneous allelic mu