This work describes the development of a new diagnostic tool to assess the severity of rheumatoid arthritis (RA) using infrared image collection and analysis. Early work showed that the temperature distribution of joints of hands and knees of patients with RA was statistically significantly different from that of normal subjects. Current work identified ankles as also significant for an assessment of RA. Moreover, the patients were classified in three levels of RA severity (High, Medium, and Low) using a C5.0 decision tree classifier with excellent results: Sensitivity (true positive cases) of 96 % and a specificity (true negative cases) of 92%. Future work will automate the image analysis and test clinically by comparing to MR as ground truth.