Exfoliative dermatitis and erythroderma in infancy are rare. Clinicians need to be alert to the possible diagnosis of Omenn's syndrome (OS), a rare form of combined immunodeficiency in infants presenting with exfoliative dermatitis, erythroderma, recurrent infections, eosinophilia and raised IgE. OS is fatal unless treated by bone-marrow transplantation (BMT). We describe a 3-week-old girl who presented with a widespread scaly erythematous rash and stomatitis, and was initially treated for presumed atopic eczema and primary herpes stomatitis. Aged 3 months, she developed erythroderma, diarrhoea and hepatosplenomegaly associated with eosinophilia, raised serum IgE and low IgG, IgA and IgM levels, abnormal lymphocyte populations and skin histology, consistent with a diagnosis of OS. She remains well 16 months after a human leucocyte antigen-matched bone-marrow transplant from an unrelated donor.