In 32 patients from 27 families affected with hereditary neophropathy (Alport syndrome) the glomerular basement membranes were examined electron microscopically and the percentage of characteristically split and thin basement membrane portions was determined. The clinical course was more severe in males which corresponded with a higher rate of basement membrane alterations: on an average in males 61% split and 6% thin but only 18% split and 21% thin in females. The splitting lesion increased with age in males but not so in females. There were also indications for a possible positive correlation of the splitting lesion and the grade of proteinuria. Compared with the splitting lesion basement membrane thinning seemed to be of minor importance.