Radionuclide imaging with gallium is commonly performed in the evaluation of patients with suspected Pneumocystis carinii pneumonia (PCP) and is known to be highly sensitive for detection of PCP. We present a patient with acquired immunodeficiency who developed PCP in the bases of both lungs while on aerosolized pentamidine prophylaxis. A gallium scan demonstrated focal uptake in the lung bases, a pattern generally not associated with PCP, and was extremely useful in guiding bronchoscopy. An aerosol ventilation scan performed after complete resolution of the clinical illness demonstrated prominent ventilatory defects in the lung bases corresponding to the regions of previous gallium uptake. We speculate that the underlying ventilatory abnormality may have contributed to poor drug delivery to the lung bases. Pneumocystis carinii pneumonia must be considered with any focal area of gallium accumulation in patients receiving aerosolized pentamidine.