Previous research on Japanese union members has documented occupational differences in employees' attitudes toward union organizations (Japan Productivity Center, 1990; Morishima, 1993). Yet, little has been done to examine why these differences occur. This study examines, using a data set obtained in 1987 from about 4000 employees, the correlates of union loyalty across white-collar and blue-collar occupations, and among white-collar occupations. A model is developed from previous research on employees' attitudes toward union organizations and on Japanese industrial relations. The results obtained by OLS analyses of union loyalty correlates-members' judgements of union contributions to improvements in employment outcomes and of employers' human resource policies-indicate that Japanese unions have benefited mainly blue-collar workers and traditional white-collar workers, such as those in administrative positions, relative to those in technical positions.