Generally, menarche is considered an indicator of girls' sexual maturation. In Thailand, this study on the menarcheal age in female pupils was conducted in Bangkok, and the central, northern, northeastern and southern regions of Thailand, which differ in their geography, tradition and customs. In carrying out the research, 8,200 questionnaires were distributed to female pupils aged 10-17 years. The result was that the mean menarcheal age was 12.3 years. For the length of menstrual period, it was found that 83.7 per cent had a period of less than 7 days and 75.7 per cent used 2-4 pieces of sanitary napkins per day. For the regularity of menstrual period, 48.5 per cent had a regular period and 41.8 per cent had a period once within 2 months. For the persons whom these female pupils needed for advice about menarche, the study indicated that mothers were mostly needed. For the mean menarcheal age reported in each region, it was found that in the central region the mean menarcheal age was 12.5 years, the North was 12 years, the Northeast was 12.4 years and the South was 12.7 years. These data prove to be statistically significant, that is the lowest menarcheal age of female pupils was in the North while the highest was in the South. In terms of nature and characteristics of female pupils' menstruation in 4 regions there was no difference. When comparing the BMI value of female pupils already having a period in each region, the result was that there was no difference. The BMI value of those already having a period was higher than those not having had a period; and in the latter group, the BMI value of female pupils in the North was the highest while that in the Central region was higher than in the Northeast and the South.