The deceleration phase of the pitching mechanism requires forceful eccentric contraction of the posterior rotator cuff. Because traditional isotonic strengthening may not be specific to this eccentric pattern, a more effective and functional means of strengthening the posterior rotator cuff is needed. Twelve collegiate baseball pitchers performed a moderate intensity isotonic dumbbell strengthening routine for 6 weeks. Six of the 12 subjects were randomly assigned to an experimental group and placed on a Theraband(R) Elastic Band strengthening routine in a functional-diagonal pattern to emphasize the eccentric contraction of the posterior rotator cuff, in addition to the isotonic routine. The control group (n = 6) performed only the isotonic exercises. Both groups were evaluated on a KIN-COM(R) isokinetic dynamometer in a functional diagonal pattern. Pretest and posttest average eccentric force production of the posterior rotator cuff was compared at two speeds, 60 and 180 degrees /s. Data were analyzed with an analysis of covariance at the .05 level with significance at 60 degrees /s. Values at 180 degrees /s, however, were not significant. Eccentric force production at 60 degrees /s increased more during training in the experimental group (+19.8%) than in the control group (-1.6%). There was no difference in the two groups at 180 degrees /s; both decreased (8 to 15%). Theraband was effective at 60 degrees /s in functional eccentric strengthening of the posterior rotator cuff in the pitching shoulder.