To investigate how communication among physicians, patients, and family/companions influences patients' decision making about participation in clinical trials. We video recorded 235 outpatient interactions occurring among oncologists, patients, and family/companions (if present) at two comprehensive cancer centers. We combined interaction analysis of the real-time video-recorded observations (collected at Time 1) with patient self-reports (Time 2) to determine how communication about trial offers influenced accrual decisions. Clinical trials were explicitly offered in 20% of the interactions. When offers were made and patients perceived they were offered a trial, 75% of patients assented. Observed messages (at Time 1) directly related to patients' self-reports regarding their decisions (2 weeks later), and how they felt about their decisions and their physicians. Specifically, messages that help build a sense of an alliance (among all parties, including the family/companions), provide support (tangible assistance and reassurance about managing adverse effects), and provide medical content in language that patients and family/companions understand are associated with the patient's decision and decision-making process. In two urban, National Cancer Institute-designated comprehensive cancer centers, a large percentage of patients are not offered trials. When offered a trial, most patients enroll. The quality and quantity of communication occurring among the oncologist, patient, and family/companion when trials are discussed matter in the patient's decision-making process. These findings can help increase physician awareness of the ways that messages and communication behaviors can be observed and evaluated to improve clinical practice and research.