Although the therapeutic alliance is a robust predictor of psychotherapy outcomes, less is known about specific factors that influence its development. The present study examined the association between patient-rated alliance and several patient interpersonal factors (distress, rigidity, & style) in the context of a randomized clinical trial comparing cognitive-behavioral therapy (CBT) and interpersonal therapy (IPT) for bulimia nervosa. Using hierarchical linear modeling, the study found that early and middle alliance quality were both negatively associated with patients’ baseline interpersonal distress and positively associated with baseline interpersonal affiliation. Middle alliance quality was also predicted by interactions between treatment group and rigidity, treatment group and affiliation, and treatment group and control. Overall, the rate of alliance growth was higher in IPT than in CBT. Using group-based trajectory analysis, the study found three divergent patterns of alliance development in the sample (high & improving, low & improving, and low & stable) and detected group mean differences between two of the trajectory groups in terms of patient interpersonal distress and hostile-submissiveness.