A retrospective study was carried out to determine the relationship between parity and bone mineral density (BMD) in middle-aged women. Eight hundred and twenty-five woman aged 41-76 years were recruited from four general practice registers in Cambridge. Subjects were unselected as to their health status. Each subject completed a detailed health questionnaire. Participation rate was 50%. The main outcome measure was BMD measured at the spine (L2-4, n = 825) and hip (neck, intertrochanter and Ward's triangle; n = 817) by dual-energy X-ray absorptiometry (DXA) using the Hologic QDR-1000 densitometer. It was found that the unadjusted mean BMD was significantly higher at all sites among the parous women (p = 0.031 to < 0.00001), and remained significantly higher at the femoral neck (p = 0.025), intertrochanter (p = 0.001) and Ward's triangle (p = 0.045) after adjusting for age and body mass index (BMI). Similar findings were seen after stratifying for potential confounding variables. There was a consistent upward trend of BMD with increasing parity at all sites. Parity remained a significant independent predictor of BMD at all sites after controlling for age, BMI, menopausal status, oral contraceptive and hormone replacement therapy use, smoking status and breast-feeding status in multiple linear regression analyses. There was, on average, a 1.0% increase in BMD per live birth. Our findings therefore suggest a positive relationship between parity and bone mass.