Periductal mastitis and mammary duct ectasia are now considered as separate disease entities in the female breast, and these two diseases affect different age groups and have different etiologies and clinical symptoms. These two entities have very rarely been reported in the male breast and they have long been considered as the same disease as that in the female breast without any differentiation. We report here on the radiologic findings of a rare case of periductal mastitis that developed during the course of chemotherapy for lung cancer in a 50-year-old male. On ultrasonography, there was a partially defined mass with adjacent duct dilatation and intraductal hypoechogenicity, and this correlated with an immature abscess with a pus-filled, dilated duct and periductal inflammation on the pathologic examination.