Despite increasing interest in how voice assistants like Siri or Alexa might improve health care delivery and information dissemination, there is limited research assessing the quality of health information provided by these technologies. Voice assistants present both opportunities and risks when facilitating searches for or answering health-related questions, especially now as fewer patients are seeing their physicians for preventive care due to the ongoing pandemic. In our study, we compared the 4 most widely used voice assistants (Amazon Alexa, Apple Siri, Google Assistant, and Microsoft Cortana) and their ability to understand and respond accurately to questions about cancer screening. We show that there are clear differences among the 4 voice assistants and that there is room for improvement across all assistants, particularly in their ability to provide accurate information verbally. In order to ensure that voice assistants provide accurate information about cancer screening and support, rather than undermine efforts to improve preventive care delivery and population health, we suggest that technology providers prioritize partnership with health professionals and organizations.