We report a patient with systemic vasculitis who developed a Pneumocystis carinii pneumonia whilst he was on steroid n cyclophosphamide treatment. Severe lymphopenia was noted with a CD4 count of 0.023 x10(9)/1 with no evidence of Human Immunodeficiency virus infection. A literature review of recent reports about this particular topic reveals that the development of lymphopenia in patients under immunosuppressor treatment is a fact of a paramount importance. In that sense, a periodic monitorization of both the granulocyte and the lymphocyte count is widely recommended in patients under cytotoxic treatment. A dose adjustment or even suppression of drug administration would be necessary if severe lymphopenia was observed.