A measles outbreak occurred in California [USA] in the 1st half of 1975, especially in the San Francisco Bay area. Of 4 adult patients with complicated cases, 2 were receiving immunosuppressive chemotherapy, and both died from a giant-cell pneumonia. The clinical presentation in such cases may be atypical, and special viral isolation and immunofluorescent techniques may be diagnostically helpful. A significant (P < .0005) trend toward the occurrence of measles in adolescents was observed in this epidemic, which may also occur in future epidemics elsewhere. With widespread but incomplete immunization, it is possible for the nonimmunized to reach adulthood without acquiring natural infection. As a result, internists and pediatricians must consider the possibility of measles and be aware of its serious potential in the immunosuppressed patient.