The effect of L-thyroxine (25 g) and lacidipine (2 mg) on the microcirculation in SLE patients was studied by comparing pre- and post-treatment laser Doppler flow (LDF) parameters. We found that the drugs caused impairment of LDF measures. This can be attributed to endothelial dysfunction, platelet activation, or a "steal phenomenon". A significant negative correlation of platelet count and LDF parameters was observed.