Left heart failure has been studied in 50 patients over a period of 2 1/2 years in order to determine the criteria of early x-ray diagnosis. By evaluating the hilar areas, the pulmonary veins, the parenchyma of the lungs, the costophrenic regions, and the contour and size of the heart, the authors were able to suggest the diagnosis of left heart failure in the earliest stage. The signs were consistent and diagnostic. They were informative in patients who had only vague complaints and atypical or minimal physical findings. They were also useful in assessing the reponse to treatment and detecting early recurrence.