A longitudinal-linkage analysis approach was developed and applied to an outbredpopulation. Nonlinear mixed-effects models were used to describe the lactationpatterns and were extended to include marker information following single-markerand interval mapping models. Quantitative trait loci (QTL) affecting the shapeand scale of lactation curves for production and health traits in dairy cattlewere mapped in three U.S. Holstein families (Dairy Bull DNA Repository familiesone, four, and five) using the granddaughter design. Information on 81informative markers on six Bos taurus autosomes (BTA) was combined with milkyield, fat, and protein percentage and somatic cell score (SCS) test-dayrecords. Six percent of the single-marker tests surpassed the experiment-wisesignificance threshold. Marker BL41 on BTA3 was associated with decrease in milkyield during mid-lactation in family one. The scale and shape of the proteinpercentage lactation curve in family four varied with BMC4203 (BTA6) allele thatthe son received from the grandsire. Some map locations were associated withvariation in the lactation pattern of multiple traits. In family four, themarker HUJI177 (BTA3) was associated with changes in the milk yield and proteinpercentage curves suggesting a QTL with pleiotropic effects or multiple QTL inthe region. The interval mapping model uncovered a QTL on BTA7 associated withvariation in milk-yield pattern in family four and a QTL on BTA21 affecting SCSin family five. The developed approach can be extended to random regressions,covariance functions, spline, gametic and variance component models. The resultsfrom the longitudinal-QTL approach will help to understand the genetic factorsacting at different stages of lactation and will assist in positional candidategene research. Identified positions can be incorporated into marker-assistedselection decisions to alter the persistency and peak production or thefluctuation of SCS during a lactation.