[영문]In most countries, women are absent from work more frequently than men. Using personnel data, we find that the absences of women below the age of 45 follow a 28-day cycle, while the absences of men and of women over the age of 45 do not. We interpret this as evidence that the menstrual cycle increases female absenteeism. To investigate the effect on women's earnings, we use a simple model of statistical discrimination. Consistent with the model, we find absenteeism has a more negative effect on men's earnings and this difference declines with seniority. The increased absenteeism induced by the 28-day cycle explains at least 14 percent of the earnings gender differential.