Early menarcheal age and menstrual irregularities are commonly associated with high body mass index (BMI)1,2. Menarcheal age of 14 years and over has also been linked with prolong and irregular cycles3. The impact of BMI on the age at menarche and duration of the menstrual cycle in normal (NM), overweight (OW) and obese (OB) young school girls in south-south Nigeria was examined. Participants (n = 52; median age = 20 years) were classified into 3 groups: NM (n = 27; BMI = 21.0 ± 2.0 Kg/m2), OW (n = 14; BMI = 28.1 ± 1.2 Kg/m2) and OB (n = 11; BMI = 31.5 ± 0.6 Kg/m2). BMI was calculated from the equation; BMI = weight (Kg)/height (m2)4. Cross-sectional data on menarche and menstrual cycle history were obtained through self-administered questionnaires. Approval and consent were obtained from the school authorities and participants before the study commenced.Data analysis showed a statistical significant difference in the BMI (p < 0.0001). Though the mean (± SD) ages at menarche were similar (NM = 13.0 ± 1.2 years; OW = 12.9 ± 0.9 years; OB = 13.7 ± 0.9 years) (p > 0.05), the average duration of the menstrual cycle (NM = 27.3 ± 1.4 days; OW = 27.9 ± 0.6 days; OB = 29.4 ± 1.4 days) and menstrual bleeding (NM = 4.4 ± 0.6 days; OW = 4.4 ± 0.8 days; OB = 5.4 ± 0.9 days) differed significantly according to the BMI (p < 0.0001 and p < 0.05 respectively). There was an increase in the average length of the menstrual cycle with increased BMI (r = 0.515, 95% CI = 0.28 – 0.69, p < 0.0001). However, the correlation between BMI and age at menarche was weak (r = 0.2527, 95% CI = -0.02 – 0.49, p > 0.05). The average duration of the menstrual cycle tend to decrease with older menarcheal age, though this was not statistically significant (r = -0.016, 95% CI = -0.29 – 0.26, p > 0.05).A higher than normal BMI may be an important risk factor for longer menstrual cycles and menses (menstrual irregularities), with adverse consequences for future reproductive and overall health of young females.