Introduction: Despite numerous studies on risk factors for osteoporosis the effect of breast-feeding on bone mineral density (BMD) is unclear. In this study our aim was to determine the influence of total duration of breast-feeding on BMD, and subsequent risk of osteoporosis. Methods: A total of 1,486 postmenopausal women over the age of 40 were included in the study. Women with diseases or who were under drug treatments known to affect bone metabolism were excluded. The BMD of the lumbar spine and femoral neck were measured by dual-energy X-ray absorptiometry technique, using either the Norland XA-26 or Lunar DPX-IQ densitometers and were transferred to standard values. Patients were placed in groups of five with respect to the duration of their breast-feeding as never, 1-12 months, 12-24 months, 24-60 months, > 60 months. Results: One-way ANOVA test showed a significant difference between the lumbar spine and femoral neck BMD of these groups of women (p < 0.001, p < 0.001). Post hoc Bonferroni correction revealed that both the lumbar spine and femoral neck BMD results of women with longer duration of total breast-feeding were significantly lower than those of women with less duration of total breast-feeding. In subsequent analysis other potential risk factors were also considered in a multiple linear stepwise regression model. Years since menopause (p < 0.001), weight (p < 0.001), total duration of breast-feeding (p < 0.001), and body mass index (p=0.001) were found to be the most important predictors for lumbar spine BMD; and age (p < 0.001), weight (p < 0.001), years since menopause (p < 0.001), and total duration of breast-feeding (p < 0.001) for femoral neck BMD. Conclusions: This study showed significant associations between total duration of breast-feeding and BMD. In conclusion, total duration of breast-feeding might be an important risk factor besides age, weight, and years since menopause in postmenopausal osteoporosis.