To monitor the extent and spread of the epidemic of measles that began in New Zealand in 1991, a special surveillance system was established. Reports from 13 area health boards were received on a weekly basis; the Bay of Plenty Area Health Board provided data aggregated over several weeks on a less frequent schedule. Nine thousand two hundred thirty-nine measles cases were reported during the 6-month period July-December 1991, with the highest incidence being reported from the Tairawhiti (1,027 cases/100,000 population) and Taranaki (558/100,000) Area Health Boards; South Island areas reported rates that were all below the national average of 280/100,000. Of the 8,684 cases for which information on age was obtained, over one-third (2,957, or 34%) occurred among children less than 5 years of age, most of whom had not been vaccinated. The failure to vaccinate a sufficiently high percentage of children against measles is the major factor that contributed to this epidemic.