['To study the clinical course and changes in antineutrophil cytoplasmic antibody (ANCA) titers in patients with ANCA associated vasculitis (AAV) who developed opportunistic infections.', 'Among the patients with AAV tested in the Immunopathology Laboratory at the Massachusetts General Hospital between 1989 and 1998, all patients who experienced opportunistic infections (n = 16) were included. We retrospectively studied their clinical features and examined the relationship between changes in ANCA titer and the onset of the opportunistic infections. ANCA titers were measured by antigen-specific ELISA.', 'Of the 16 AAV patients with opportunistic infection, 15 had no evidence for active vasculitis at the time of the infection. Among these 15 patients, opportunistic infections were associated with a steep fall in ANCA titers. There was no consistent pattern of change in C-reactive protein levels. In 7 patients, the immunosuppressive regimen was increased for new clinical findings shortly before the diagnosis of an opportunistic infection, despite the absence of histologic documentation of active vasculitis. Three of these 7 patients died. One patient, who did not experience a significant fall in ANCA titer. i.e., less than 4-fold from his prior peak, was simultaneously found to have Pneumocystis carinii pneumonia and biopsy proven active vasculitis.'] Are opportunistic infections preceded by a rapid fall in antineutrophil cytoplasmic antibody ( ANCA ) titer in patients with ANCA associated vasculitis?