Antineutrophil cytoplasmic antibodies (ANCA) are of established value in the diagnosis of Wegener's granulomatosis allowing early introduction of therapy. These patients are at risk of opportunistic infection, especially whilst receiving immunosuppressive drugs and this may mimic reactivation of disease. We present three cases of Wegener's granulomatosis complicated by opportunistic infection and assess the value of ANCA detection. Two presented with symptoms compatible with disease reactivation but ANCA were negative. One died with pulmonary infection due to Pneumocystis carinii, Aspergillus fumigatus and Herpes simplex. Transbronchial biopsy in the second case revealed Pneumocystis carinii. A third case had strongly positive serum ANCA at diagnosis but in addition pulmonary infection with Legionella pneumophila. ANCA detection is of value in patients with Wegener's granulomatosis, but the result must be interpreted in the full clinical context.