The objective of the present study was to determine median age at menarche using the status quo method and the independent effects of ethnic background and socioeconomic status on the age at menarche in Peruvian girls from two distinct levels of altitude: Lima (150 m) and Cerro de Pasco (4,340 m). The sample included 503 girls from Lima and 625 girls from Cerro de Pasco, ages 10-18 years. Ethnic background was determined by four parental surnames. Subjects were classified as Quechua when one or more surnames were from Quechua origin and Spanish when four surnames were from Spanish origin. Socioeconomic status was determined using a socioeconomic index score and subjects were classified as belonging to the middle-low, low, or very low class. Median age at menarche was calculated by survival analysis using the Life Table Method with a 95% Confidence Interval (CI). Among Quechua and Spanish girls, age at menarche occurred later at high altitude than at sea level. Median ages at menarche for the overall groups were 13.08 (12.91-13.25, 95% CI) years in Lima and 14.33 (14.17-14.48, 95% CI) years in Cerro de Pasco (P < 0.0001). Socioeconomic status had no significant effect on menarcheal age (P > 0.05). Controlling socioeconomic status in the design, median age at menarche was still higher at high altitude than at sea level (P < 0.05). After logistic regression analysis, an effect on age at menarche of chronological age and place of residence was observed, but not by ethnic background, socioeconomic status and not by interactions of age and place of residence, age and socioeconomic status, place of residence and socioeconomic status, place of residence and ethnic background, or socioeconomic status and ethnic background. Thus later age at menarche observed in girls at high altitude is not due to differences in ethnicity or socioeconomic status. © 1996 Wiley-Liss, Inc.