This paper presents the application of a classification of anal fistulas to an unselected consecutive series of 793 patients treated for this condition at St Mark's Hospital from 1968 to 1973 inclusive. The fistulas were divided into five categories on their anatomical relationships: superficial (16 per cent), intersphincteric (54 per cent), trans-sphincteric (21 per cent), suprasphincteric (3 per cent) and extrasphincteric (3 per cent), with the remaining cases multiple or unclassified (3 per cent). There was good correlation between the categories and presentation, physical signs and treatment of the fistulas. Two hundred and forty-nine patients had potentially difficult fistulas. Follow-up of these patients revealed healing of almost all the fistulas, but the functional results were less satisfactory (incontinence of loose stool in 17 per cent and of flatus in 25 per cent, soiling in 31 per cent).