To determine in patients with systemic lupus erythematosus (SLE) the relationships among serum total and free prolactin (PRL) levels, isoforms of PRL and lupus activity. In a cross-sectional study, 259 patients with SLE were tested for serum total PRL, serum free PRL, and PRL in fractions obtained after gel filtration chromatography (in 70 sera taken at random) by immunoradiometric assay based on disease activity. A significant correlation between direct PRL and free PRL levels was found in patients with and without lupus activity (r = 0.475, P