An assessment of the incidence of predisposition to psychological disorders in flying personnel showed that two-thirds of individuals who failed to withstand the stress of flying were predisposed to nervous breakdown. As this group undoubtedly contained individuals capable of adapting themselves satisfactorily to operational flying, it was decided that only severely predisposed individuals should be rejected at entry, and that those with other degrees of predisposition should be watched carefully during training and eliminated if signs of temperamental unsuitability appeared. Surveys of psychological disorder in air crew showed similar findings year by year: about 3,000 cases of nervous breakdown and 300 of lack of confidence annually, which indicated that a uniform standard of psychiatric examination was being maintained. Types of nervous breakdown were chiefly anxiety and hysteria, both together accounting for over 90 per cent of cases. Practically all cases of nervous breakdown (98.4 per cent) arose from underlying psychological rather than physical causes.