Pulmonary complications occurred in 38 of 212 recipients of renal allografts since August 1967. Eighteen of the patients ultimately died. Six clinical patterns could be discerned: (1) Pulmonary edema, which occurred early and was associated with renal malfunction, was evidenced by fever, weight gain, hypertension, and creatinemia. (2) Interstitial pneumonia secondary to viral infections was heralded with high spiking fever and delayed respiratory symptoms. Cytomegalovirus was deemed responsible for most cases of interstitial pneumonia. (3) Bacterial pneumonia occurred acutely with fever, systemic toxicity, and sputum production. (4) Fungal pneumonia and (5) Pneumocystis carinii pneumonia are now quite rare as primary diseases, although both may be found in patients dying of bacterial pneumonia. (6) Pulmonary embolus is rarely evident in its classic form but may be associated with thrombophlebitis or pyelonephritis of the transplanted kidney.