Background Cancer is the second-leading cause of death in the United States. Clinical trials translate basic science discoveries into treatments needed by cancer patients. Inadequate accrual of trial participants is one of the most significant barriers to the completion of oncology clinical trials. Objective The purpose of this study was to investigate trial-level factors that affect accrual and/or completion of oncology clinical trials, identify gaps in the literature, and indicate opportunities for future research. Design A systematic review of the literature on trial-level factors that affect accrual and/or completion of oncology clinical trials was performed. Searches in PubMed and Scopus identified 6582 studies. Based on eligibility criteria, 16 studies were selected for the review. Results were analyzed according to the following: a) background factors, b) disease-related, c) treatment-related, and d) trial design. Results Background factors that were investigated in relation to oncology clinical trial accrual and/or completion included sponsor, number and location of participating institutions, competing trials, time of trial opening, and fast-track status. Disease-related factors included the annual incidence and type(s) of targeted cancer. Several types of treatment such as drugs, radiation and surgery were examined in the studies. Trial design factors included trial development time, eligibility criteria, randomization, sample size, trial phase, placebo use, and required protocol procedures and their timing. Conclusion With low patient participation rates in oncology clinical trials that hold promise for future treatments, it is imperative that trial-level factors affecting accrual be identified and addressed to facilitate the completion of trials.