Summary:SCID is a heterogeneous group of disorders characterized by defective T cell and B cell function. Eczematous and morbilliform eruptions are common, and graft-versus-host disease (GVHD) due to maternal engraftment has been documented. We sought to better characterize SCID-related cutaneous disease observed prior to BMT and to compare the eruption to conventional GVHD. Medical records of 51 patients with SCID treated between 1982 and 1999 were reviewed. Ten of 51 (20%) had rash and evidence of maternal engraftment prior to BMT (study group). Eleven of 51 (22%) had no rash or evidence of engraftment pre-BMT but developed GVHD following transplant (control group). Skin biopsies were available for review for 8/10 of the study group and for 8/11 of the control group. Cutaneous findings consisted of a scaling, erythematous maculopapular eruption spread widely over the trunk and extremities, with near-erythroderma in some patients. Microscopically, biopsies from the study group differed significantly from controls. Key differences included parakeratosis (P р 0.01), psoriasiform hyperplasia (P р 0.04) and spongiosis (P р 0.04). The dermatopathologic findings of transplacental GVHD differ from the pattern of post-transplant GVHD. A 'psoriasiform-lichenoid-spongiotic' pattern with necrotic keratinocytes should trigger consideration of SCID and maternal engraftment in the dermatopathologic evaluation of eruptions of infancy. Bone Marrow Transplantation (2001) 28, 227-233. Keywords: severe combined immunodeficiency disease (SCID); bone marrow transplant (BMT); graft-versus-host disease (GVHD); maternal engraftment Severe combined immunodeficiency diseases (SCIDs) are a heterogeneous group of genetic disorders characterized by deficiency of both T and B cell functions. The mode of