We retrospectively examined the sensitivity and specificity of gallium lung scans for detecting Pneumocystis carinii pneumonia in 22 patients with known or suspected acquired immune deficiency syndrome. Correlations were made between bronchoscopic results and gallium scan findings interpreted using a simple system (normal or abnormal) and a graded score (1 to 4). All 12 patients with Pneumocystis had abnormal scans by both interpretations (sensitivity, 100%). Five of these 12 patients had normal arterial PO2 values, chest roentgenograms, or both. In the 10 patients without Pneumocystis, scans were read as abnormal in 8 using the simple system (specificity, 20%) but were abnormal in only 1 using the grading system (specificity, 90%). Five patients with Pneumocystis pneumonia had scans and bronchoscopy after treatment; neither method of interpretation correlated with the presence of organisms. We conclude: (1) gallium scanning is a sensitive initial diagnostic test in patients with suspected Pneumocystis pneumonia; (2) a graded scoring system improves specificity; (3) an abnormal gallium scan (3 or greater) in patients with suspected Pneumocystis pneumonia is an indication for biopsy, even when the PO2 and/or chest roentgenogram are normal.