In aiming to define the characteristics of HIV-infected subjects developing neutropenia and describe the causes, features and effects of neutropenia we undertook a retrospective study in a dedicated HIV unit in London, UK. Two hundred and forty-four patients with documented neutropenia, defined as absolute neutrophil count (ANC) < 1,000/mm3, during a 12-month period were studied. First neutropenia occurred at a median CD4 count of 30 cells/mm3. Low CD4 count was associated with longer episodes of neutropenia with a more profound nadir. Two-thirds of episodes lasted less than 2 weeks. ANC nadir was < 500 cells/mm3 in 45% of episodes. Infections were most frequent in patients with profound but brief neutropenic episodes. Neutropenia was generally mild, short-lived and associated with late-stage disease. However, profound neutropenia did develop suddenly in some patients with no prior history of neutropenia (in 13% first neutropenic ANC recorded was < or =500 cells/mm3), and at CD4 count > 200 cells/mm3. Most patients were receiving multiple myelosuppressive therapies. Infection was associated with brief, profound neutropenia.