The authors examined the longitudinal relationship of patient-rated perfectionism, clinician-rated depression, and observer-rated therapeutic alliance using the latent difference score (LDS) analytic framework. Outpatients involved in the Treatment for Depression Collaborative Research Program completed measures of perfectionism and depression at 5 occasions throughout treatment, with therapeutic alliance measured early in therapy. First, LDS analyses of perfectionism and depression established longitudinal change models. Further LDS analyses revealed significant longitudinal interrelationships, in which perfectionism predicted the subsequent rate of depression change, consistent with a personality vulnerability model of depression. In the final LDS model, the strength of the therapeutic alliance significantly predicted longitudinal perfectionism change, and perfectionism significantly predicted the rate of depression change throughout therapy. These results clarify the patterns of growth and change for these indicators throughout depression treatment, demonstrating an alternative method for evaluating longitudinal dynamics in therapy.