In a cohort of HIV-infected patients, this study compares the clinical and immunological features at the time of AIDS diagnosis of patients who either received primary Pneumocystis carinii prophylaxis (P+; n = 335) or who did not (P-; n = 289). Frequency of P carinii pneumonia was lower in P+ than in P- patients (14.9% vs 26.0%; p < 0.001). Conversely, toxoplasmic encephalitis, esophageal candidiasis, cytomegalovirus disease and M avium complex disease were more frequent in P+ patients. CD4+ count (median/mm3) at the time of AIDS diagnosis was lower in P+ than in P- patients: 22 vs 97 (p < 0.001); this suggests that early intervention delays the onset of AIDS for about one year. While searching for new prevention strategies against other opportunistic infections, efforts should be expanded to improve prophylaxis of P carinii pneumonia which remains in France the most frequent first AIDS-related illness.