Cannabis use among individuals with cancer is best understood using survey self-report. As cannabis remains federally illegal, surveys could be subject to nonresponse and measurement issues impacting data quality. We surveyed individuals using medical cannabis for a cancer-related condition in the Minnesota Medical Cannabis Program (MCP). Although survey responders are older, there are no differences by race and ethnicity, gender, or receipt of reduced cannabis registry enrollment fee. Responders made a more recent purchase and more recently completed an independent symptom assessment for the registry than nonresponders, suggesting some opportunity for nonresponse error. Among responders, self-report and MCP administrative data with respect to age, race, gender, registry certification, and cannabis purchase history were similar. Responders were less likely to report receipt of Medicaid than would be expected based on registry low-income enrollment eligibility. Although attention should be paid to potential for nonresponse error, surveys are a reliable tool to ascertain cannabis behavior patterns in this population.