Nut samples of hazelnut (Corylus avellana L.) cultivars, selections, and seedlings were cracked and scored for frequency of good kernels and 7 types of defects. Blanks, poorly filled nuts, and moldy kernels were the most common defects. Variance components were calculated using defect counts from trees of 17 genotypes which were sampled in each of 4 years. The largest components were for among genotypes, genotype × year interaction, and between samples within trees. Year effects were large only for poorly filled nuts. The genotype × year interaction term was large for black tips, shrivels, and blanks. Variance components indicate limited genetic variability for brown stain, but moderate to large amounts for all other defects. Narrow sense heritability estimates from regression of progeny means on mid-parent values exceeded 50 % for frequency of blanks (51 %), moldy kernels (61 %), kernels with black tips (60 %), and doubles (84 %), but was low for frequency of shrivelled kernels (22 %), poorly filled nuts (25 %), and brown stain (15 %). The low heritability of poorly filled nuts appears to indicate that the frequency of poor fill in the first crop or two produced by a young seedling does not reliably predict the genotype's susceptibility to this disorder rather than a lack of additive genetic variance. Heritability was intermediate for frequency of good kernels (42 %). Phenotypic correlation coefficients indicate that nut weight, kernel percentage, fiber rating, and pellicle removal rating are, for applied breeding purposes, inherited independently of the frequency of the 7 types of defects.