The overall prevalence of Hudson-Stahli line in a group of 550 subjects was 65 per cent with bilateral involvement and a further 4 per cent with unilateral involvement; analysis of two subgroups, with either blue/grey iris colour or with brown/green colour, suggested that the incidence is slightly higher in the latter group, particularly in the third to fifth decades. Irrespective of iris colour, the incidence rises with age. Both the length and the density of the line increase with age. For a given age, brown/green eyes have longer lines than blue/green eyes, but similar densities. Concordance of length and density between the right and left eyes of a pair is seen in about 80 per cent of subjects.