This study provides a quantitative assessment of heritability of size of the normal physiologic cup when estimated as a continuous variable. A horizontal cup-disc ratio was estimated clinically among a sample of 37 pairs of monozygotic (MZ) and 26 pairs of like-sexed dizygotic (DZ) twins. Differences in intrapair variance and intraclass correlation coefficient between the MZ and the DZ samples were large and highly significant, and suggest that inheritance plays a major role in determining diameter of the normal physiologic cup. Values of heritability index (Hl, an estimate of the proportion of variance within pairs explained by genetic factors) ranged from 0.60 to 0.80 by various computational methods employed.