The purpose of this study was to analyze the effects of a quadriceps femoris muscle eccentric training program on strength gain in patients with patellar tendinitis. The effect of an eight-week eccentric exercise program on quadriceps femoris muscle work was evaluated in four groups of subjects--two groups of "normal" (healthy) subjects and two groups of patients with patellar tendinitis. All four groups participated in a home muscle stretching exercise program, but only two groups--one group of normal subjects (N-A) and one group of subjects with tendinitis (T-A)--received additional eccentric training on an eccentric isokinetic dynamometer. The eccentric quadriceps femoris muscle work ratio (involved limb/uninvolved limb x 100) was used to quantify strength in the N-A and T-A Groups. Pain ratings were recorded for subjects with tendinitis before and after the eight-week experiment and were correlated with the dependent variable using a Spearman rank-order correlation coefficient. The N-A Group performed significantly better than all subjects with tendinitis (p less than .05). Subjects in the T-A Group, however, showed a trend toward increasing eccentric quadriceps femoris muscle work capacity over the eight-week training period. As pain ratings in the T-A Group increased, work ratios decreased. We concluded that eccentric exercise may be an effective treatment for patellar tendinitis, but that knee pain may limit optimal gains in strength.