Preventive services are delivered at rates far below recommended levels. Although lack of time has frequently been cited as an important factor, little is known about how much time primary care clinicians devote to prevention and how they prioritize that time. Work sampling was used to estimate the proportion of time spent on prevention during routine care of patients by primary care clinicians in two hospital-based clinics serving indigent patients. Clinicians were prompted by computer at random intervals to describe their current activity and, if the activity was prevention-related, to choose the specific activity from a list modified from the U.S. Preventive Services Task Force (USPSTF) recommendations. Proportions of time spent on prevention overall and by specific prevention activity were calculated, and the association between USPSTF ratings of specific prevention activities and proportion of time spent on those activities was examined using Kendall's Tau. Clinicians in these clinics spent just 11% of their time on prevention, or about 7 minutes per patient per year. Screening for just two diseases, breast cancer and cervical cancer, accounted for half of all prevention-related activity. There was no overall relation found between proportion of time by specific prevention activity and USPSTF ratings. Thus, the primary care clinicians spent little time on prevention and did not apportion that time according to USPSTF recommendations. If these results are representative, time constraints in actual practice may be too severe to deliver the full range of preventive services suggested by USPSTF.