This report is an analysis of five ethnic groups in New York City (Jews, blacks, Puerto Ricans, Italians, and Irish), and makes correlations between ethnicity, social class and mental illness. It estimates the extent to which five indicators of health in area populations account for variation in rates of mental hospitalization for men and women (21-64). It is an assessment of the relative importance of social class and ethnicity in accounting for rates of hospitalization. Among the findings of the study are the following: (1) there is a trend for greater outpatient utilization by blacks, (2) diagnosed mental illness is found more often among Puerto Ricans than among the rest of the population, (3) the lower rates of psychiatric hospitalization of Jews is associated with demographic and cultural characteristics, (4) while the Italians' rate of mental illness is low, it has increased for both sexes in the last 65 years, (5) compared to other first and second generation Americans and to the total white population, the Irish have higher rates of diagnosed mental illness and probably higher prevalence rates of alcoholism in the community as well. In this study, the knowledge of the ethnic composition of certain geographic areas (with social class held constant) is used to predict with reasonable accuracy close to 37% of the incidence of psychiatric hospitalization. These studies cumulatively suggest that social support generated by the ready accessibility of ethnically similar people plays an important role in influencing vulnerability to stress and psychiatric disorder. (Author/AM)