OBJECTIVE To study bronchial responsiveness to inhaled histamine among HIV-infected patients. METHODS A prospective study in a regional infectious diseases unit. METHODS Three groups of patients were studied. Group A consisted of AIDS patients (n = 7) who had had Pneumocystis carinii pneumonia (PCP), group B of AIDS patients (n = 7) not known to have had PCP, and group C of asymptomatic HIV-positive patients (n = 7). Inhalational histamine challenge in cumulative doses (0.03-3.91 mumol) was administered by a nebulizer. It was stopped when the forced expiratory volume in 1 sec (FEV1) had fallen by more than 20% of the baseline value or when the cumulative dose administered exceeded 3.91 mumol. Response was measured as percentage change in FEV1 from the baseline value, and plotted on a linear scale against log dose histamine to enable the dose of histamine causing a 20% fall in FEV1 (PD20-FEV1) to be determined. Statistical analysis was performed by analysis of variance. RESULTS AIDS patients previously infected with PCP (group A) had a significantly lower PD20-FEV1 [(mean, 0.31 mumol; range, 0.07-0.95; s.d., 0.31; s.e., 0.12; 95% confidence interval (CI), 0.03-0.60)] than AIDS patients without PCP (group B; mean, 1.01 mumol; range, 0.20-2.00; s.d., 0.67; s.e., 0.25; 95% CI, 0.39-1.64) or asymptomatic HIV-positive patients (group C; mean, 1.28 mumol; range, 0.49-1.80; s.d., 0.51; s.e., 0.19; 95% CI, 0.81-1.76) (P < 0.05). There was no significant difference between groups B and C. All patients recorded PD20-FEV1 within the asthmatic range of bronchial hyper-responsiveness. CONCLUSIONS These results suggest that development of PCP in a small group of HIV-infected patients induces a significantly greater degree of bronchial hyper-responsiveness.