Physicians and other health-care professionals are rapidly adopting personal digital assistants (PDA). Palm pilots and other hand-held computers are also increasingly popular among medical students. PDAs can be used for medical student education and physician training, daily clinical practice, and research. PDAs and their increasing integration with information technology in hospitals could change the way health care is delivered in the future. But despite the increasing use of PDAs, evidence from well-designed research studies is still needed to show how much these devices can improve the quality of care, save patients' lives, and ultimately reduce health-care expenses. In this Review of PDA use in health care, the operating systems, basic functionality, security and safety, limitations, and future implications of PDAs are examined. A personal perspective and an introduction to medical PDA applications, software, guidelines, and programmes for health-care professionals is also provided.