Measles cases reported to the Centers for Disease Control from 1985 to 1989 were analyzed to determine the characteristics of measles cases transmitted in medical settings. A total of 1,209 medial setting cases were identified, which represented 3.5% of all reported cases. Of medical setting cases, 66% (795) were in known or presumed patients and 28% (341) were in health care workers. The largest groups of health care workers with measles were nurses (101, 29.6%) and physicians (65, 19.1%). Health care workers acquired measles from patients (90.6%) and other health care workers (9.4%), and transmitted measles to patients, other health care workers, and family members. Of 333 (97.7%) health care workers with known measles vaccination status, 232 (68.0%) were eligible for vaccine; only 46 (19.8%) had received a documented dose. Twenty-nine percent of health care workers with measles were born before 1957, older than the age for routine measles vaccination. The relative risk of measles for physicians and nurses was 8.4 (95% confidence interval [CI], 6.6, 10.8) and 2.1 (95% CI, 1.8, 2.7) respectively, compared with nonhealth care workers of the same ages. In 1989 the Immunization Practices Advisory Committee (ACIP) recommended that health care workers be required to document two doses of measles vaccine or other evidence of measles immunity at the time of employment. Implementation of ACIP recommendations for health care workers and appropriate isolation precautions for known and suspected patients with measles could reduce the transmission of measles in medical settings.