Dynamic and static gadolinium-diethylenetriaminepentaacetic acid(Gd-DTPA)-enhanced magnetic resonance imaging (MRI) were evaluated as measures of joint inflammation in arthritis, by a comparison with macroscopic and microscopic signs of synovitis. Furthermore, the importance of the size of the evaluated synovial areas was investigated, as was the optimal time for enhancement measurements. Seventeen rheumatoid arthritis knees and 25 osteoarthritis knees, scheduled for arthroscopy or arthrotomy, were included. Macroscopic and microscopic synovial inflammation as well as nine histologic tissue characteristics were graded at four preselected biopsy sites. Preoperative T1-weighted dynamic fast low angle shot and static spin-echo Gd-enhanced MRI were performed. The dynamic enhancement rate and the static enhancement were measured in the entire synovial membrane of a preselected slice as well as at the four biopsy sites, and compared to synovial pathology. The rate of early enhancement of the total synovial membrane of the preselected slice, determined by dynamic MRI, was highly correlated with microscopic evidence of active inflammation (Spearman p = 0.73; p < 10(-7). Dynamic MRI could distinguish knees with and without synovial inflammation with a high predictive value (0.81-0.90). Moderate and severe inflammation could not be differentiated. The early enhancement rate was correlated with histologic features of active inflammation, particularly vessel proliferation and mononuclear leucocyte infiltration. Dynamic evaluation of small synovial sections at the biopsy sites and static spin-echo MRI resulted in considerably weaker correlations to histologic inflammation than dynamic evaluation of the total synovium. The optimal time for enhancement measurements was one-half to one minute after Gd injection, as the highest correlation coefficients to histologic inflammation were observed in this interval. Dynamic MRI can be used to determine synovial inflammation. Evaluation of large synovial areas one-half to one minute after Gd injection best reflects joint inflammation.