Post-treatment cancer care is often fragmented and of suboptimal quality. We explored factors that may affect cancer survivors' post-treatment care coordination, including oncologists' use of electronic technologies such as e-mail and integrated electronic health records (EHRs) to communicate with primary care physicians (PCPs). We used data from a survey (357 respondents; participation rate 52.9%) conducted in 2012-2013 among medical oncologists caring for patients in a large US study of cancer care delivery and outcomes. Oncologists reported their frequency and mode of communication with PCPs, and role in providing post-treatment care. Seventy-five per cent said that they directly communicated with PCPs about post-treatment status and care recommendations for all/most patients. Among those directly communicating with PCPs, 70% always/usually used written correspondence, while 36% always/usually used integrated EHRs; telephone and e-mail were less used. Eighty per cent reported co-managing with PCPs at least one post-treatment general medical care need. In multivariate-adjusted analyses, neither communication mode nor intensity were associated with co-managing survivors' care. Oncologists' reliance on written correspondence to communicate with PCPs may be a barrier to care coordination. We discuss new research directions for enhancing communication and care coordination between oncologists and PCPs, and to better meet the needs of cancer survivors post-treatment.