Two recent epidemiological studies (PROS and NHANES III) from the USA noted earlier sexual maturation in girls, leading to increased attention internationally to the age at onset of puberty. We studied the timing of puberty in a large cohort of healthy Danish children in order to evaluate differences between USA and Denmark, as well as to look for possible secular trends in pubertal development. Healthy Caucasian children from public schools in Denmark participated in the study which was carried out in 1991-1993. A total number of 826 boys and 1,100 girls (aged 6.0-19.9 years) were included, and pubertal stages were assessed by clinical examination according to methods of Tanner. In boys testicular volume was determined using an orchidometer. We found that age at breast development 2 (B2) was 10.88 years, and mean menarcheal age was 13.42 years. Girls with body mass index (BMI) above the median had significantly earlier puberty (age at B2 10.42 years) compared with girls with BMI below the median (age at B2 11.24 years, p <0.0001). Similarly, menarcheal age was significantly lower in girls with BMI above the median compared with girls with BMI below the median (13.12 vs. 13.70 years, p = 0.0012). In Danish boys we found that age at genital stage 2 (G2) was 11.83 years. Both sexes were significantly taller compared with data from 1964, but timing of pubertal maturation seemed unaltered. Finally, puberty occurred much later in Denmark compared with recent data from USA. We could not detect any downwards secular trend in the timing of puberty in Denmark between 1964 and 1991-1993 as seen in the US. Obesity certainly plays a role in the timing of puberty, but the marked differences between Denmark and USA cannot be attributed exclusively to differences in BMI. A possible role of other factors like genetic polymorphisms, nutrition, physical activity or endocrine disrupting chemicals must therefore also be considered. Therefore, we believe it is crucial to monitor the pubertal development closely in Denmark in the coming decades.