We believe that this century will be the century of quality of life (QOL). Both medical prediction and QOL prediction are indispensable in clinical practice. Therefore, we investigated how to predict QOL outcomes after various therapeutic interventions using our customized questionnaire. Seventy-four cataractus patients treated with day surgery (S1), 106 undergoing renal transplantation (S2), 30 with pacemaker treatment (S3), and 36 elderly persons supported by government-mandatory nursing/healthcare service for persons aged >65 years (S4) participated in this study. Our self-administered questionnaires and the Life Satisfaction Index (LSI) were at first compared. Pearson's correlation coefficients between our questionnaires and LSI were S1, r=0.92 (P<0.05); S2, r=0.69 (P<0.01); and S3, r=0.87 (P<0.01). Cronbach's α coefficients of our questionnaires were high enough for acceptance for clinical use. Our questionnaires contained 11 (S1), 11 (S2), 9 (S3), and 12 (S4) main factors and the cumulative contributions were 0.82, 0.77, 0.85, and 0.81, respectively. Whereas all patients whose baseline QOL was low before interventions exhibited improved total QOL after interventions, most patients whose total QOL was high before interventions underwent impaired total QOL thereafter. Divergent points were identified for QOL improvement or deterioration after therapeutic interventions according to each disorder investigated. Significant negative correlations were noted between baseline QOL and QOL changes. Our QOL prediction theory may be helpful when discussing interventions with individual patients in a range of therapeutic settings.