Our research was undertaken to determine the median age of natural menopause and correlates of the timing of menopause in Spain and the United States (U.S.). A population-based sample of 300 women from Madrid, Spain and a random sample of 293 women from Fallon Community Health plan (FCHP), a health maintenance organization (HMO) in central Massachusetts, were interviewed using a semi-structured questionnaire. Logit analysis and logistic regression were used to estimate the median age at menopause and identify factors associated with it. The median age of natural menopause in Spain is estimated at 51.7 years, and in the U.S., it is 52.6 years. In Spain, women with any children (OR = 0.58, 95% CI: 0.25, 1.36) and a lower body mass index (BMI) (OR = 0.45, 95% CI: 0.27, 0.78) had later ages at menopause while current smokers (OR = 5.51, 95% CI: 1.82, 16.7) had earlier ages of menopause in a multivariate model. A multiplicative interaction between smoking status and parity was identified, and an interaction term included in the multivariate model (OR = 0.58, 95% CI: 0.35, 0.94). In the U.S., household income, marital status, and education level were statistically associated with age at natural menopause in bivariate models. These factors were no longer statistically significant after adjustments in a multivariate model. Oral contraceptive use, cycle length, and cycle regularity were not statistically associated with the age of menopause in either country. The ages of natural menopause in Spain and the U.S. are comparable to other industrialized nations. The factors associated with the timing of natural menopause, in particular smoking and BMI, are consistent with those identified in previous studies.