We studied the effect of 1-year transdermal estrogen replacement therapy (ERT) on bone mineral density (BMD) and biochemical markers of bone turnover in osteopenic postmenopausal systemic lupus erythematosus (SLE) patients in a randomized, double-blind, placebo-controlled trial. SLE patients were randomly allocated to treatment (estradiol; 50 microg transdermal 17beta-estradiol; n=15) or placebo ( n=17) group. Both groups received 5 mg continuous oral medroxyprogesterone acetate, 500 mg calcium and 400 IU vitamin D(3). L(1)-L(4) spine (LS), left femur and total hip BMD were measured at baseline and at 6 and 12 months. Serum osteocalcin (OC) and degradation products of C-terminal telopeptides of type-I collagen (CTx) levels were measured at baseline and 3, 6, 9, and 12 months. There was a significant difference in the percentage change of LS BMD at 6 months between the two groups (103.24+/-3.74% (estradiol group) vs 98.99+/-3.11% (placebo group); P<0.005). There was a significant decrease within the estradiol group in the CTx levels between baseline and all subsequent visits ( P<0.05). There was no significant difference in SLE disease activity index, Systemic Lupus International Collaborating Clinics/American College of Rheumatology (ACR) damage index and corticosteroid dose during the study period. Transdermal estradiol may prevent bone loss in postmenopausal SLE women at the lumbar spine and femur, with no increase in disease activity among postmenopausal SLE women receiving transdermal ERT. The high dropout rate (8/15) leads us to the conclusion that efficacy of HRT in a high-risk group such as SLE women can be attained only in a small number of patients, provided all inclusion/exclusion criteria are strictly adhered to.