Peripheral blood mononuclear cells (PBMC) and synovial fluid mononuclear cells (SFMC) from patients with chronic rheumatoid arthritis (cRA) (n = 15) and mononuclear cells from PBMC from early rheumatoid arthritis (eRA) (n =5) and healthy volunteers (HV) (n = 10) were examined by flow cytometry for expression of CD4, CD14, CD19, CD45RO, OX40, and OX40L. Closed squares = peripheral blood from patients with cRA; semi-solid squares = peripheral blood from patients with eRA; gray squares = synovial fluid from patients with cRA; white squares = peripheral blood from healthy volunteers. We observed an increased percentage of OX40- and OX40L-expressing cells in the synovial fluid, with CD4 + CD45RO + T-cells being more positive for OX40 and CD19+ B cells and monocytes being more positive for OX40L. Paired data (PBMC and SFMC) were analyzed by Wilcoxon signed-rank test, non-paired data were analyzed by the Mann-Whitney U-test. Solid lines represent median with interquartile range and whiskers represent the 5th to 95th percentiles. *P <0.05 and **P <0.01, ***P <0.001.