Electronic mail has been in use for almost 20 years at Boston's Beth Israel Hospital and is an integral part of the clinical information system. Through a study of usage patterns during a one-week period, we found that 1247 persons sent 7482 messages to 1302 different recipients. Each category of user (attending physician, house officer, nurse, etc.) sent the most e-mail to others of the same user category. Through an electronically administered questionnaire, we found that self-reported usage patterns had a high correlation (r = 0.6) with measured use. Sixty-six percent of respondents used e-mail daily or weekly, and 58% used it for issues of patient care; nearly all users found this useful for communicating about patient care issues. Ninety percent of respondents felt e-mail made their lives easier and 61% felt it had a humanizing influence on their lives. We conclude that the e-mail system is well-utilized by clinical personnel and felt to be useful in both patient care and nonpatient care situations.