It is generally accepted among dermatologists that certain skin diseases are subject to seasonal influence. This belief, however, is based primarily on anecdotal observation rather than fact-based evidence. To address this controversy, we performed a retrospective analysis of a total of 3931 subjects enrolled in 7 phase 3 clinical trials across the diseases acne, atopic dermatitis, and seborrheic dermatitis and found a seasonal effect to exist in the treatment response of the 3 very common dermatologic diseases studied. To be truly representative of a drug's benefit when the disease is known to be impacted by seasonality, efficacy figures in the package insert should therefore be based on data collected from patients enrolled across all seasons.