Sex hormones are possible determinants of systemic lupus erythematosus (SLE). Hence treatment with tamoxifen, which competes for oestrogen receptors, was assessed in 11 female patients with stable SLE in a double-blind crossover trial. The indices used included clinical signs of SLE, renal function, leucocyte counts, serum antinuclear and anti-DNA activity, and serum levels of complement and immune complexes. No patient improved on tamoxifen and two deteriorated. Significant side effects were encountered. The trial yielded no evidence that tamoxifen had an ameliorative effect on clinical or laboratory indices of activity of SLE.