During two consecutive winter seasons (1985 and 1986) Auckland, New Zealand, experienced epidemic rates of Group A meningococcal disease, a pattern not previously recognized in New Zealand. The overall rate was 8.3/100,000/year. The highest annual rate (64.7) occurred in children 0 to 23 months of age. A city-wide vaccine campaign commencing in May, 1987, was conducted over 6 weeks among children 3 months to 13 years of age with special emphasis on reaching populations at highest risk (Maori and Pacific Island Polynesian children in certain geographic regions of Auckland). Children from 2 to 13 years of age received a single dose of monovalent Group A meningococcal vaccine. Children ages 3 to 23 months received two doses at least 1 month apart. Overall approximately 130,000 doses were delivered; coverage was approximately 90% in the single dose target group. Among the younger children approximately 89% received the primary dose. Only approximately 26% received the recommended "booster" dose. After 2 1/2 years of active surveillance (1987 to 1989) there were no cases of invasive Group A meningococcal disease in children appropriately vaccinated for age. In contrast to this 100% efficacy the efficacy of a single dose of monovalent Group A meningococcal vaccine to prevent illness in the youngest children during the 1987 epidemic period was 52% (95% confidence interval (-330%, 95%)) falling to 16% (95% confidence interval, (-538%, 90%)) after 1 year. Four cases that occurred in infants 3 to 7 weeks before the scheduled "booster" campaign supports limited true efficacy. However, the prescribed 1 to 3-month interval between the two doses in infants may be too long.(ABSTRACT TRUNCATED AT 250 WORDS)