Objective. To assess red blood cell deformability (RCD) in patients with rheumatoid arthritis (RA) without extraarticular manifestations and in RA with vasculitic complications (RV), and to assess whether in vitro dipyridamole improves RCD. Methods. An improved filtration technique was used to measure RCD in 15 patients with RA, 18 patients with RV, and 20 age- and sex-matched normal control subjects. Washed erythrocytes suspended in buffer, at 5% hematocrit, were filtered through 4.7µ Nuclepore Hemafil PC membranes. The initial steady-state relative filtration pressure (iRFP) was used as an index to assess RCD. A lower iRFP value reflects increased deformability, a higher value reflects a decrease. For each sample, 2 cell suspensions were prepared, one blank (control) and one containing 5 µM dipyridamole. Results. The mean iRFP values of cells obtained from patients with RV were significantly higher than those of cells obtained from normal controls. There were no appreciable differences in iRFP between RA patients and normal controls. When the erythrocytes were pretreated in vitro with 5 µM dipyridamole before filtration, their deformability improved markedly (iRFP values were reduced) in all study subjects, compared with untreated cells. Conclusion. RCD is reduced in patients with RV, and treatment with dipyridamole may be beneficial if reduced RCD contributes to impaired microvascular perfusion.