Recommendations to limit the working hours of house staff are forcing directors of training programs to reevaluate how house officers spend their time. We studied how 35 house officers in internal medicine spent their on-call time in three teaching hospitals: an urban county hospital, a university hospital, and a regional Veterans Administration medical center. Trained observers accompanied each member of different on-call teams for five nights and quantified how their time was spent. Teams consisting of residents and interns admitted three new patients per night at the Veterans Administration hospital, six at the university hospital, and eight at the county hospital. Each house officer received 16 to 25 calls per night. Up to 12 percent of their time was spent doing procedures (such as inserting intravenous catheters or drawing blood specimens) most of which could have been done by nonphysicians. From 87 to 175 minutes of on-call time was spent in direct patient evaluation, and the mean time spent on each new-patient evaluation ranged from 17 to 31 minutes. The mean time before the evaluation was interrupted ranged from 7 to 11 minutes. In contrast, 66 to 197 minutes per night was spent documenting new-patient evaluations in the hospital record. The average sleep time ranged from 122 to 273 minutes; however, the mean time before sleep was interrupted ranged from 40 to 86 minutes. We conclude that while on call, house officers spend relatively little time in direct patient contact, but they spend considerable time charting. They are frequently interrupted while working and trying to sleep. These data may be useful in finding administrative ways to improve patient care and the experience of the house staff while on call and in evaluating the effect of reforms.