In the last 60 years, there have been approximately 50 studies of various suspected risk factors associated with acute necrotizing ulcerative gingivitis. Two thirds of these studies have been either surveys or case reports; nearly all the rest have been case-control studies. There have been only a few longitudinal and population-based studies, most of which were in high-risk populations. The findings of many studies pertain to gingivitis in general rather than in acute necrotizing ulcerative gingivitis in particular; the findings of other studies are based on small numbers or solely on case histories without the inclusion of control subjects. There are comparisons between the epidemiology of acute necrotizing ulcerative gingivitis as it occurs in military populations and the epidemiology of meningococcal meningitis (98-103). Both diseases have been reported in closed communities such as in young recruits away from home and in new surroundings. A temporal trend upward in disease frequency was described for both infections during World War II. Both infections appear to be characterized by low communicability, with no documented transmission by fomites or vectors. Furthermore, active disease in both cases is associated with markedly increased numbers of normally indigenous flora: B. intermedius in the case of acute necrotizing ulcerative gingivitis and Neisseria meningococcus in the case of epidemic meningococcal meningitis. Risk factors associated with both diseases include crowding, physical fatigue, increased stress, low socioeconomic status, and failure of host defense mechanisms. In general, there is potential for longitudinal studies of young people such as new military recruits and college students who are undergoing the transition from dependence to independence. It is intriguing that these young adults are suddenly at increased risk of this disease in late adolescence when they should be at peak fitness and, just as suddenly, at decreased risk after their early thirties. It appears that the dynamic tension between bacterial agents, environmental stressors, and host defense mechanisms is abruptly altered in certain young people, allowing the characteristic lesions to develop. Furthermore it appears that, in the United States at least, the disruption is limited primarily to white people relative to black people. It is possible that disease develops because young adults are brought together from diverse locations and perhaps exposed to a new microbial agent. However, acute necrotizing ulcerative gingivitis has not been demonstrated clearly to be transmissible, and if it is, the mode of transmission is not known.(ABSTRACT TRUNCATED AT 400 WORDS)