Graft versus host disease is frequently encountered in patients with an allogenic bone marrow transplantation. The disease apparently results from the activity of the donor T lymphocytes which react against the recipient's cells. There are two categories of graft versus host disease: the acute form which occurs within 3 months of the graft and the chronic form which occurs thereafter. Skin is the predominant site of manifestations, although the liver, the gut and the eye may be involved. In acute graft versus host disease, there is a characteristic maculo-papular rash raising a difficult differential diagnosis which pathology examination of biopsy cannot always resolve. The chronic disease is easier to recognize on the basis of local or generalized lichenoid or sclerodermal skin lesions. Several risk factors have been identified including the degree of donor-recipient HLA mismatch, recipient age and the number of T cells grafted. Graft versus host disease leads to immune deficiency with lymphoid depletion and increased risk of infection. Treatment depends on the site of organ involvement and extension. Corticosteroids and immunosuppressors are used.