A platelet count was made in 37 patients with Progressive Systemic Sclerosis (PSS), 6 with Sjögren's Syndrome, 11 with Rheumatic Polymyalgia with or without Horton's Arteritis (PMR-AH), 26 with Ankylosing Spondylitis (A.Sp.), 29 with Psoriatic Arthritis, 15 with Gout and in 65 healthy subjects. In this last group the mean platelet count was 215.692/mm3, S.E. 4.167. In the various groups of patients the following mean platelet counts were determined: PSS 242.108 +/- 11.766; Sjögren 245.000 +/- 22.620; PMR-AH 276.818 +/- 25.577; A.Sp. 272.846 +/- 14.124; Psoriatic Arthritis 245.833 +/- 9.374; Gout 265.333 +/- 24.628. Statistical analysis showed a significant difference (P less than 0.05), (P less than 0.01) or (P less than 0.001) between each group of patients and the controls. However, the platelet count in the majority of patients in each group was not higher than 300.000/mm3. Statistically significant correlations were found between platelet count and some biolgoical inflammation parameters. All patients with platelets higher than 300.000/mm3 showed a high disease activity. Yet some patients with marked inflammation did not present thrombocytosis. According to literature data thrombocytosis in Rheumatoid Arthritis is considered to be correlated to disease activity. The results of this study indicate that even in other rheumatic diseases thrombocytosis is often present, apparently correlated to the inflammation. Therefore thrombocytosis is an inflammation parameter; but it is less sensitive than other ones.