BACKGROUND Age at menarche has been used as a marker of environmental conditions during childhood. Previous work has shown trends of decreasing age at menarche throughout the 19th century, but reported trends in the 20th century have been less consistent. The nature of the relationship between age at menarche and adult life anthropometric measures may be important in understanding the importance of this measure on disease in later life. OBJECTIVE To establish whether mean age at menarche changed during the first half of the 20th century, and to determine the nature of associations between age at menarche and anthropometric measures in young adulthood. METHODS 3433 female students, who were born between 1919 and 1952 and who attended health checks at the student health service of the University of Glasgow between 1948 and 1968. RESULTS Mean age at menarche decreased from 13.2 years in the earliest born to 12.5 years in the latest born students. These results were not explained by changes in socio-demographic factors. Menarcheal age was positively associated with height and negatively associated with weight and BMI, results independent of socio-demographic and behavioural factors. CONCLUSIONS The falling age at menarche described here may be related to nutritional influences in the first half of the 20th century. The influence of menarche on BMI in early adulthood may have important health consequences.