Acne is a common disease with an underlying hormonal basis; however, there has never been a study to determine the ways in which the different stages of the menstrual cycle affect acne in women. Four hundred female participants, aged 12 to 52 years, were questioned whether their acne got worse before, during, or after their menstrual period and also asked whether it was unrelated to the menstrual period. Their age, severity of acne, ethnicity, and oral contraceptive use were also recorded. Overall 177 of 400 (44%) of those interviewed experienced premenstrual flares of their acne. Severity of acne, ethnicity, and oral contraceptive use did not affect the premenstrual flare rate. Women older than 33 years had a higher rate of premenstrual flares relative to women aged 20 to 33 years (P =.03 by chi(2) analysis). We concluded that almost half of all women experience premenstrual flares of their acne. Premenstrual flares may be more common in older women.