The relation between body weight, height and the commencement of menstruation was studied in 5324 girls aged 12--15 yr attending secondary schools in the province of Pordenone. The mean age of menarche was 12 2/12 yr and varied little throughout the province. Weight and height appeared closely related to menarche. A threshold of 49.2 kg and 157.3 cm was noted. Attainment of these critical values was apparently the factor that influenced the age at which menstruation commenced.