The focus of this study is the investigation of the relation between patients' interpersonal problems, therapists' attachment representations, and the development of the therapeutic alliance over time. The authors investigated weekly alliance ratings of 281 psychotherapy inpatients, treated by 12 psychotherapists. Alliance quality was measured with the Inpatient Experience Scale. Multilevel regression models showed that patients' interpersonal problems were associated with the level of alliance quality. Therapists' attachment security was not related to alliance development, but higher attachment preoccupation of therapists was associated with lower levels of alliance quality. In addition, an interaction effect between therapists' degree of attachment preoccupation and patients' interpersonal problems explained variations of the alliance development curve over time. Limitations of the study are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved).