Despite worldwide efforts to promote HIV counseling and testing, rates of testing in Tanzania remain low. This study uses a Discrete Choice Experiment, a survey method increasingly used by health economists for the design of patient-centered health care options, to identify HIV testing preferences among two high-risk populations and evaluate, in a pragmatic randomized controlled trial (RCT), the effect of a preference-based HIV counseling and testing intervention on testing uptake in the Kilimanjaro region.