OBJECTIVE This study was done to determine the effect of multiparity and prolonged lactation on bone mineral density (BMD). METHODS This cross-sectional study included 196 perimenopausal and postmenopausal women aged 40 to 60 years old. Age, body mass index (BMI), menopausal status, duration of menopause, parity and total duration of lactation, nutritional history were recorded. Lumbar spine (LS; L2-L4) and femur neck (FN) BMD were measured using dual energy X-ray absorptiometry. Correlation of parity and lactation with BMD were investigated using multiple regression analysis. RESULTS Parity was inversely correlated to BMD for LS (β = -0.266, P = 0.001) and FN (β = -0.380, P = 0.000). This relation remained significant even after adjusting for age, BMI and duration of menopause. Duration of lactation was inversely correlated with BMD for LS (β = -0.271, P = 0.001) but no for FN (β = -0.124, P = 0.130). CONCLUSIONS Multiparity and prolonged lactation have negative impact on BMD especially with in a socioeconomic group whose nutritional intake is borderline. Our data support that parity and duration of lactation can be associated with future osteoporosis.