A retrospective cohort study was conducted to quantitate the relationship between neutropenia and rates of clinical bacteraemia among adults with HIV infection receiving medical care at one institution between 1991–5. The primary exposure, absolute neutrophil count (ANC), was summarized as mean ANC within a given week, using a five-level stratification (reference > 1000/μl). ANC stratum-specific rates of bacteraemia were calculated, by organism type. Linear trend tests were performed to assess dose-response relationship between neutropenia and rates of bacteraemia. The cohort included 1645 patients contributing 26785 patients-weeks and 191 episodes of bacteraemia. The unadjusted effect of neutropenia was most evident for bacteraemia due to E. coli (RR 3.4), Klebsiella pneumoniae (RR 16·7), and P. aeruginosa (RR 10.4). For bacteraemia due to any of these three organisms (47 episodes), with reference ANC > 1000/μl, relative rates were: 751–1000/μl, 1·12; 501–750/μl, 2.11; 251–500/μl, 13·58; 0–250/μl, 21·89.