We used Cox proportional hazards regression models with age as the timescale to estimate the relative risks and 95% confidence intervals associated with a history of benign gynecological disease. We tested the proportional hazards hypothesis graphically by using log-log survivor plots and by adding an interaction term between each time-dependent variable and time in our model. We controlled for phototype factors, including hair color (blond, red, chestnut, brown, or dark), skin complexion (fair or dark), number of nevi (very many, many, few, or none), number of freckles (very many, many, few, or none), and skin sensitivity to sun exposure. Regarding the latter, we asked participants about their skin response if exposed to the sun for the first time in summer and recorded the answers as highly sensitive, moderately sensitive, and not sensitive. We further adjusted for BMI (≤ 25 or > 25), parity (nulliparous, 1 or 2, 3 or 4, or ≥ 5 children), use of oral contraceptives (ever or never), age at menarche (< 13, 13 or 14, or ≥ 15 years), duration of menstrual cycles (irregular, ≤ 24, 25-31, or ≥ 32 days), and age at menopause (premenopausal, < 48 years, 48-51 years, or ≥ 52 years). Data on history of benign gynecological diseases and BMI were analyzed as time-dependent variables. Missing data for age at diagnosis of benign gynecological disease were imputed to the age when the subject answered the questionnaire in which the corresponding disease was declared. Missing values in age at menopause were imputed to the median age at artificial menopause (47 years) or at natural menopause (51 years) in our cohort. Missing BMI values were imputed to the BMI provided in the closest questionnaire. For all other adjustment factors, we imputed missing values to the modal category. Two-sided maximum-likelihood tests were performed in all Cox models, with P < .05 being the threshold of statistical significance. All analyses were performed with SAS statistical software (version 9.1; SAS Institute Inc, Cary, North Carolina).