Gynecomastia occurring in three men aged 44, 51, and 60 years was found to be associated with bronchogenic carcinoma. In the first patient the primary complaint was painful swelling of the knees, and the gynecomastia was noted two years later when the patient was hospitalized. In the second patient the gynecomastia was the initial symptom. It developed in the third patient weeks after thoracotomy at which an unresectable carcinoma of the lung had been found. Assessment of the patients' endocrine status, including determinations of 24-hour excretion of estrogens and of 1 7-ketosteroids, failed to identify any one endocrine factor that would explain the gynecomastia in all patients.