The National Cancer Institute Cancer Screening Research Network is launching a pilot study (Vanguard) to determine feasibility of successful completion of a clinical trial of multicancer detection tests. This focus group study reports perceptions of primary care physicians and laypersons of different clinical trial designs and willingness to participate in a multicancer detection clinical trial. We undertook 14 focus groups with 88 laypersons and 6 focus groups with 45 primary care physicians. Participants were shown graphics of clinical trial designs and asked for their reactions. Focus group recordings were transcribed verbatim, and thematic analysis of the transcripts were conducted to identify emergent themes. Primary care physician and layperson participants recognized the importance of conducting clinical trials to determine the clinical utility of multicancer detection tests. Primary care physicians expressed reluctance to participate in trials because of workload burden, and laypersons expressed hesitancy about enrolling in the control group. Primary care physicians and laypersons expressed concern about a study design in which multicancer detection test results would not be returned to the control group (intended effect), but they respectively indicated a willingness to refer patients to, or participate in, a multicancer detection test clinical trial given transparent and clear communication on collection and use of biospecimens and data, particularly if a multicancer detection test would eventually be run and results eventually returned. This study yielded important insights to guide trial design in planning prospective evaluation of multicancer detection testing. Maintaining transparency and trust while possibly withholding multicancer detection test results to maximize trial feasibility and efficiency is of particular concern.