To evaluate the effect of altitude on adolescent growth and development, three groups of healthy, well-nourished youth of similar socioeconomic status and ethnic grouping who resided at sea level (n = 1262 subjects), mid-altitude (n = 1743 subjects), and high altitude (n = 1137 subjects) were studied. The following parameters were evaluated: weight and height in all subjects; genital and pubic hair development in the males; and the initiation of breast development and age of menarche in the females. At mid-altitude, puberty appears to start at a similar age, but lasts longer, than at sea level. At high altitude, puberty starts significantly later and is more prolonged than at sea level and mid-altitude. Our data reveal relatively little effect of altitude upon growth and weight-gain patterns or final attained heights and weights. © 1988.