We examined 486 bronchoalveolar lavages (BAL) including 32 from AIDS patients with pulmonary infiltrates and 20 from patients with leukemia or after transplantation. Mycoplasmas were found in 4/32 (12.5%) HIV-positive patients compared to 4/454 (< 0.9%) HIV-negative patients (p < 0.001). All of these four HIV-positive patients suffered from advanced infection (CD4 counts < 100/microL) and developed complications (Pcp, n = 2, recurrent bacterial pneumonia, n = 1, pulmonary Kaposi sarcoma, n = 1). No mycoplasmas were detected in 20 immunosuppressed patients with leukemia or after transplantation. Our data indicate that AIDS patients may be more often colonised or infected by mycoplasmas than HIV-negative patients or other immunocompromised persons. Although the etiological role of mycoplasmas for pulmonary infections in these patients remains unclear, the finding of mycoplasmas was associated with rapid progress and development of severe complications in our study.