To test the hypothesis that pupil achievement is inversely related to class size, a 5-year study (1959-1964) examined the relationship between class size and pupil achievement in reading and arithmetic. Data were taken from records of the Baltimore public school system for all 16,449 grade 3 pupils in the class of 1959, comprised of 6,568 regular white pupils, 8,341 regular nonwhite pupils, 441 special education white pupils, and 1,099 special education nonwhite pupils. Pupils in smaller classes in both the regular and special education curricula were found to make significantly greater achievement gains than students in larger classes. Smallest class size (1-25) was considerably more productive for nonwhite than for white students. Other variables correlated to class size and pupil achievement included pupil home mobility, parental occupation and level of education, percentage of nonwhite faculty, faculty knowledge, and faculty experience. A review of related research and a description of the research design are included. Extensive comparisons from the study's findings are tabulated and described. (JK)