On the basis of a sample of 13,558 persons which is obtained through questionnaires submitted in connection with the Health Insurance Plan of Greater N. Y., a study was made of the religio-cultural background of New York City''s population. This sample represents a ratio of slightly less than one to 597 persons in the total city population. Roman Catholics made up 47.6% of the total group, Protestants 22.8%, Jewish 26.4%, and other and not reported 3.2%. Among the non-white population the Protestant percentage rose to 80.7, while the Catholics were 13.8, others 5.5, and Jewish zero. No significant differences were noted when the population was broken down by sex. A study of religious backgrounds by age indicated a higher (56.5%) incidence of white Catholics among the under-15-year.old group as opposed to only 43.6% among the 65 and over. Curiously enough the group in which the age was not reported gave the lowest incidence of Catholics (32.9%). Among the non-white people the age distribution was reversed in that the younger group reported a higher incidence of Protestant background than the older groups.