A simple instrument has been developed for quantitative measurement of stiffness (elasticity) and maximum force of the fingers and was tested in a series of 122 out-patients with RA and 101 healthy controls. Values measured for stiffness were higher in RA patients and were scattered more widely than in controls and the mean values for force in the RA patients were lower than in controls. RA patients with an elevated ESR have significantly less strength than those with low ESR. We evaluated the influence of the time of day and weather (humidity and air temperature on the wards and outside the hospital, barometer pressure) on stiffness and force. In 13 in-patients with definite or classical RA and 12 controls, measurements were performed in the morning and afternoons, over periods of 3 weeks. In RA patients, both stiffness and force increased with outside air humidity (p less than 0.05), confirming the clinical experience that stiffness increases in damp weather. In the controls, force decreased with a higher air humidity on the wards (p = 0.05) and increased with higher barometer pressure (p less than 0.05). In the controls joint stiffness was higher in the morning than in the afternoon (p less than 0.05). In the RA group, rather unexpectedly, the reverse was found (p = 0.06), possibly explained by the time of the day the investigation was done. Only in the controls was a significant increase in force noted during the 3 weeks of investigation (p less than 0.005). No correlation was seen between force and stiffness. No correlation was found between disease activity as expressed in ESR, Hb, Rose titre, ARA grading and stiffness or force.