A counselor's ability to form an accurate and complex clinical hypothesis which models the client and guides the counseling process is an important aspect of counseling. This study was conducted using Masters level counseling students to explore clinical hypothesis formation and counseling effectiveness in a semi-natural counseling task. Each of the counseling students (N=27) acted as a counselor to one of her/his classmates for four 45-minute sessions. Data were gathered at the end of the second session. Student counselors completed a Clinical Hypothesis Rating Form and student-clients rated the counselor on the Client Rating Form-Short. The results indicated that students could construct quality hypotheses and that such hypotheses were related to clients' perceptions of the counselors. Four of the clinical hypothesis components predicted 60% of the variance in the total Client Rating Form-Short score, with each subscale having its own predictor. The findings suggest that different hypothesis components are differentially related to the counselor attributes of expertness, attractiveness, and trustworthiness. (Author/NB)