Skin biopsies from 6 patients with chronic graft-versus-host disease (GVHD) were studied ultrastructurally. The 6 patients experienced an early lichenoid phase 65-135 days after the graft and 3 of them progressed to a late sclerotic phase 200-340 days after the grafting Damage to the basal membrane and to the keratinocytes of the basal layer and low spinous layers, and presence of epidermal regenerative cells were features common to the lichenoid phase of chronic GVHD and idiopathic lichen planus. The late sclerotic phase of GVHD with persistence of basal cell injury, normal periodicity and structure of the collagen fibres and numerous active fibroblasts in the upper third of the dermis were findings that distinguished GVHD from scleroderma. Satellite cell necrosis, i.e. lymphocyte satellites of necrotic keratinocyte, was observed in the two phases of chronic GVHD. Thus at the ultrastructural level the early phase of chronic GVHD mimics lichen planus, but the late sclerotic phase is distinct from scleroderma.