A total of 65,491 lactation means of log2-transformed SCC measures were analyzed from first lactation Holstein cows in Ontario. Effects of inbreeding on SCC were estimated by a nonadditive sire and dam model that included additive, dominance, and additive by additive genetic effects and regression of lactation somatic cell score on inbreeding coefficients of the cows. Variance components were estimated using the tildehat approximation to REML. Solutions were by iteration on data. Estimates of heritability for lactation somatic cell score in the narrow sense were .165 and in the broad sense were .203. The additive by additive component (2.5% of the total phenotypic variance) was almost twice as large as the dominance component (1.3%). The regression coefficient of lactation somatic cell score per 1% increase of inbreeding was .012. The average increase of the population mean of lactation somatic cell score caused by a 10% increase of inbreeding coefficient was estimated to be 10.5% of the original phenotypic standard deviation of 1.153. The inbreeding depression was thus relatively low, but, on average, inbred animals tended to have higher lactation somatic cell score. This study provides preliminary evidence that inbreeding is related to disease prevalence in large purebred dairy populations.