Secular trends in onset of menarche and obesity were examined 14 years apart in two biracial (black-white) cohorts of girls aged 8 to 17 under study for cardiovascular risk. The first cohort (N=1,190, 64% white) was examined in 1978-1979, the second (N=1,164, 57% white) in 1992-1994. The second cohort was heavier in terms of body weight and Rohrer index (weight/height3) than the first (P<0.001), except among black girls aged 12 to 13 years. Subscapular skinfold thickness increased in the second cohort of all ages (P<0.0001), while increases in triceps skinfold were less marked. The onset of menarche occurred at an earlier age in the second cohort compared with the first cohort (P<0.0001), both in black girls (11.4+/-1.3 vs 12.3+/-1.4 years) and white girls (11.5+/-1.3 vs 12.3+/-1.3 years). Furthermore, twice as many girls in the second cohort had reached menarche by ages younger than 12 years (P<0.001). All of these obesity measures were significantly associated with the age of menarche in both cohorts (P<0.001) adjusting for height, race and age at examination. These results suggest that this secular trend toward increasing frequency of early onset of menarche may be the result of increasing obesity noted in girls of both races. Since increases in body fatness and related early onset of menarche are risk factors for disorders in adult life including cardiovascular disease and breast cancer, the secular trend in the increasing incidence of obesity throughout the United States is becoming a major public health problem.