Acne vulgaris has been linked to milk ingestion, both whole and skim milk. The milk fraction that promotes acne is unknown. Five case reports are presented of male patients aged 14 to 18 years who experienced onset of acne shortly after initiation of whey protein supplementation; 3 teenagers used the supplement for muscle building in football training and the other 2 for attempting to gain weight. All 5 patients had poor response to acne treatment regimens of oral antibiotics, topical retinoids, and benzoyl peroxide. Lesions fully cleared in 4 patients after discontinuation of whey protein supplementation, but 1 patient's acne flared after reinitiation of the whey protein supplement. Two patients did not immediately discontinue whey protein supplementation; 1 of them cleared after he discontinued whey protein during his second course of isotretinoin and 1 was lost to follow-up. Among these patients, at least 6 different brands of whey protein supplementation had been used, including whey protein shakes and reconstituted powders. Whey protein may be the fraction of dairy products that promote acne formation. Larger studies are needed to determine the mechanism of comedogenesis.