Surface tension (ST), free energy measure in surface layer, results from molecules interactions. Its value is influenced by every macro- and micromolecular substances present in solution, particularly surface active compounds (surfactants). Synovial fluid (SF) contents many such substances. Phospholipids are discussed as surface active substances in SF, protecting joints, improving their movement, lowering friction. The aim of study was the assessment of SF surface tension in 2 groups of inflammatory joint diseases – seronegative spondyloarthropathies (Spa) and rheumatoid arthritis (RA) and its relations with chosen markers of inflammation. SF from 25 patients (16 F, 9 M) with Spa and RA was evaluated for ST and total cell count (TCC), total protein and albumin concentrations. Mean ST was determined by Wilhelmy plate method (10 measurements for each sample) in the computer aided-tensiometer. Fluids were divided into 2 groups: group 1 from patients with Spa (n=6), and group 2 with RA (n=19). SF from patients with spa has significantly lower ST then from patients with RA (42.42±7.02 vs 47.99±4.59 mN/m) p=0.032. It was connected with significantly higher concentration of total proteins (5±0.49 vs 3.9±0.95 g/dL) p=0.021 and albumins (2.53±0.68 vs 1.92±0.43 g/dL) p=0.015. There was no significance in TCC between two groups of patients. ST seems to be valuable method for total assessment of compound of SF and could be suitable for differential diagnosis of type of joint disease.