Examination of large numbers of strains of meningococci associated with systemic disease in US Army personnel over a 7-year period has shown a changing pattern of sero-group prevalence and an increasing proportion of sulfadiazine-resistant organisms. These findings underline the failure of sulfadiazine prophylaxis and the necessity of applying a specific vaccine to this population. Knowledge of this kind from other populations will be required in order to provide appropriate control measures-vaccine or chemoprophylaxis.