The aims of this study were (1) to confirm previously identified quantitativetrait loci (QTL) on bovine chromosomes 6, 11, 14, and 23 in the Danish Holsteincattle population, (2) to assess the pleiotropic nature of each QTL on milkproduction traits by building multitrait and multi-QTL models, and (3) toinclude pedigree information on nongenotyped individuals to improve theestimation of genetic parameters underlying the random QTL model. Nineteengrandsire families were analyzed by single-trait (ST) and multitrait (MT) QTLmapping methods. The variance component-based QTL mapping model was implementedvia restricted maximum likelihood (REML) to estimate QTL position andparameters. Segregation of the previously identified QTL was confirmed on bovinechromosomes 6, 11, and 14, but not on 23. A highly significant (1%chromosome-wise level) QTL was found on chromosome 6, between 37 and 73 cM. ThisQTL had a strong effect on protein percentage (PP) and fat percentage (FP)according to ST analyses, and effects on PP, FP, milk yield (MY), fat yield(FY), and protein yield (PY) in MT analyses. A QTL affecting PP was detected onchromosome 11 (at 70 cM) using ST analysis. The MT analysis revealed a secondQTL (at 67 cM) approaching significance with an effect on MY. The ST analysisidentified a QTL for MY and FP on chromosome 14, between 10 and 24 cM. Theextended pedigree (nongenotyped animals) was included to estimate geneticparameters underlying the random QTL model; that is, additive polygenic and QTLvariances. In general, the estimates of the QTL variance components were smallerbut more precise when the extended pedigree was considered in the analysis.