Background. Decision aids are now a well-established means of supporting patients in their medical decision making. The widespread use of decision aids invites questions about how their components contribute to patient decisions. Objective. The objective of this study was to measure the importance of second opinions, patient-specific outcome forecasts, and patient testimonials relative to patient clinical and socioeconomic factors and the primary physician recommendation on the decision to undergo full knee replacement surgery to treat knee osteoarthritis. Methods. Middle-aged and older members of the RAND American Life Panel (N = 1616) chose whether to recommend surgery as a treatment for each of 3 hypothetical patients (vignettes) presented in a video-enhanced internet survey. Vignettes randomly sampled levels of scenario attributes. Results. Second opinions, person-specific outcome forecasts, and 2 consistent patient testimonials strongly affected respondents’ decision making; a single testimonial, however, did not significantly affect decisions. Conclusions. Information provided in a decision aid, including person-specific outcome forecasts and testimonials, can affect patient choices. The strong effect of testimonials and respondents’ interest in reviewing them reinforces concerns about unwanted influence when testimonials are biased.