The characteristics of Pneumocystis carinii pneumonia (PCP) in patients with connective tissue diseases (CTDs) were examined retrospectively. Nine patients were enrolled in this study. Their mean age was 57.1 years. All the patients received a high-dose steroid or immunosuppressant. The onset (mean 6.6 days) of fever, cough, breathlessness, and geographical ground-glass opacities revealed by chest computed tomography was acute. The serum beta-D: -glucan level increased with a simultaneous increase in the Krebs von den Lungen (KL)-6 or surfactant protein D level. The serum immunoglobulin G (IgG) and albumin levels and the peripheral blood lymphocyte count at the onset of PCP were low, but only the serum IgG level decreased significantly. The patients were treated with trimethoprim-sulfamethoxazole or pentamidine isetionate. Six patients died eventually: two patients of progressive respiratory failure, two probably due to a recurrence of the PCP, and two with microbial respiratory infections other than PCP. Five of the six patients required mechanical ventilation. Three patients received secondary prophylaxis and survived. In conclusion, the acute onset was characteristic of PCP in patients with CTDs. High-dose steroids, immunosuppressants, and hypogammaglobulinemia are risk factors; and respiratory failure requiring mechanical ventilation, severe secondary infections, and a lack of secondary prophylaxis are poor prognostic factors. Secondary prophylaxis is recommended for all of these patients.