Magnetic resonance imaging of the thorax was performed on ten occasions in eight HIV-positive patients with a clinical picture suggestive of Pneumocystis carinii pneumonia. The diagnosis of PCP was subsequently confirmed on six occasions. Patients without PCP had low MRI profusion scores, while four of six patients with PCP had MRI profusion scores greater than 6/21. Neither the chest roentgenogram appearance nor computer-generated T1 and T2 relaxation times could reliably distinguish between these two groups. Magnetic resonance imaging may be useful in the early and noninvasive diagnosis of PCP in HIV-positive patients.