Circulating levels of P- and L-selectins and the degree of T-lymphocyte activation were assessed by enzyme-linked immunosorbent assays in 75 selected patients with rheumatoid arthritis (RA), systemic sclerosis (SSc) and systemic lupus erythematosus (SLE) at various clinical stages, and in 40 healthy blood donors matched for age and gender. Mean levels of P-selectin were significantly higher than normal in RA (lower in patients with clinical remission) and SSc (higher in patients with early-onset diffuse disease), but not in SLE. In contrast, mean L-selectin levels were significantly higher than normal in SLE (no correlation to the degree of disease activity), but not in RA or SSc. Mean levels of soluble interleukin-2 receptors (sIL-2R), reflecting mainly T-lymphocyte activation, in patients with active RA, SSc and SLE were almost double the normal level; however, correlations between individual levels of circulating P- or L-selectins and sIL-2R within groups revealed a strong positive correlation only between L-selectin and sIL-2R (r = 0.66, p<0.001), and only in patients with SLE. Given the different expression of P- and L-selectins, these findings indicate a distinct pattern of immune cell activation in chronic diseases that share an overactivation of T-lymphocytes. The possible clinical value of quantitation of circulating P-selectin in patients with RA and SSc on the one hand, and L-selectin in patients with SLE on the other, should be investigated by prospective studies.