Norway has a field recording system for dairy cattle that includes recording ofall veterinary treatments on an individual animal basis from 1978 onwards.Application of these data in a genome search for quantitative trait loci (QTL)verified genome-wise significant QTL affecting clinical mastitis on Chromosome(Chr) 6. Additional putative QTL for clinical mastitis were localized to Chrs.3, 4, 14, and 27. The comprehensive field recording system includes informationon somatic cell count as well. This trait is often used in selection againstmastitis when direct information on clinical mastitis is not available. Theabsence of common QTL positions for the two traits in our study indicates thatthe use of somatic cell count data in QTL studies aimed for reducing theincidence of mastitis should be carefully evaluated.