The evidence for cannabis's treatment efficacy across different conditions varies widely, and comprehensive data on the conditions for which people use cannabis are lacking. We analyzed state registry data to provide nationwide estimates characterizing the qualifying conditions for which patients are licensed to use cannabis medically. We also compared the prevalence of medical cannabis qualifying conditions to recent evidence from the National Academies of Sciences, Engineering, and Medicine report on cannabis's efficacy in treating each condition. Twenty states and the District of Columbia had available registry data on patient numbers, and fifteen states had data on patient-reported qualifying conditions. Chronic pain is currently and historically the most common qualifying condition reported by medical cannabis patients (64.9 percent in 2016). Of all patient-reported qualifying conditions, 85.5 percent had either substantial or conclusive evidence of therapeutic efficacy. As medical cannabis use continues to increase, creating a nationwide patient registry would facilitate better understanding of trends in use and of its potential effectiveness.