Since 1983, the National Cancer Institute (NCI) has collected data by means of its Cancer Information Service (CIS), a toll-free telephone helpline for health care professionals and members of the public who have questions about cancer treatment, diagnosis, and prevention. These data reveal information about the characteristics of callers and their questions and about how inquiries reflect mass media promotions and secular trends. A request for a publication is the most common type of inquiry, followed by information about specific cancer sites, smoking prevention and cessation, other types of prevention, cancer treatment, cancer symptoms, referrals to physicians, NCI clinical trials, hospital and clinic-based screening programs, and general counseling or coping. Breast cancer is the most common cancer of interest, followed by respiratory system cancers, colon and prostate cancers, leukemia, melanoma, nonHodgkin's lymphoma, cervical cancer, general or unspecified skin cancer, and ovarian cancer. Responding to these other caller inquiries, CIS counselors may proactively guide callers to a desirable goal, such as screening mammography. Protocols have been developed to assist counselors' proactive efforts, and preliminary results are beginning to support this approach. The findings gathered in this study underscore the health education potential of telephone helplines and point to the need for controlled evaluation research on the effectiveness of proactive counselor advice.