An Index for measuring Visual Functioning on the basis of self-assessment is presented and evaluated clinically and statistically. The construct validity and the reliability are shown to be sufficient. The 'Visual Functioning Index' has been applied to a group of cataract patients. In bilateral cataract patients visual functioning is, of course, correlated to visual acuity in the best eye, but this correlation is not straight forward. So, for the assessment of visual impairment both visual acuity and visual functioning must be measured. Application of the Index to a group of monaphakic cataract patients indicates a good improvement of visual function obtained even by first cataract extraction. Ideally cataract surgery should be performed before the total visual functioning and social integration is severely damaged. On this basis about 20% of the patients in this study should have been operated at an earlier stage, which among other things suggests a need for additional surgical capacity.