Quantitative trait loci (QTL) affecting milk production and health of dairycattle were mapped in a very large Holstein granddaughter design. The analysisincluded 1794 sons of 14 sires and 206 genetic markers distributed across all 29autosomes and flanking an estimated 2497 autosomal cM using Kosambi's mappingfunction. All families were analyzed jointly with least-squares (LS) andvariance components (VC) methods. A total of 6 QTL exceeding approximateexperiment-wise significance thresholds, 24 QTL exceeding suggestive thresholds,and 34 QTL exceeding chromosome-wise thresholds were identified. Significancethresholds were determined via data permutation (for LS analysis) and chi-squaredistribution (for VC analysis). The average bootstrap confidence interval forthe experiment-wise significant QTL was 48 cM. Some chromosomes harbored QTLaffecting several traits, and these were always in coupling phase, defined byconsistency with genetic correlations among traits. Chromosome 17 likely harbors2 QTL affecting milk yield, and some other chromosomes showed some evidence for2 linked QTL affecting the same trait. In each of these cases, the 2 QTL were inrepulsion phase in those families appearing to be heterozygous for both QTL, afinding which supports the build-up of linkage disequilibrium due to selection.