The mental health of 70 school teachers for the mentally retarded, 60 direct care staff in social welfare facilities for the mentally retarded, 124 nurses and 369 general office workers was investigated by means of General Health Questionnaire (GHQ). The results were: 1) The incidence of mental unhealthiness was 44.6% among staff serving the mentally retarded, tending to be higher than among nurses, and significantly higher than among general office workers. 2) Among staff serving the mentally retarded, females tended to be more mentally unhealthy than males; among school teachers for the mentally retarded, the incidence among females was significantly higher than among males. 3) As to the incidence by age and experience, among staff serving the mentally retarded, the younger the age and the shorter the experience, the higher the incidence of mental unhealthiness.