INTRODUCTION. QTL mapping is an essential tool for the understanding of the genetic basis of complex traits, including health and production traits in farm animals and disorders and quantitative risk factors for disease in animal and human populations. QTL Express has provided a user-friendly and web-accessible analysis tool that has seen wide use for the analysis of experimental data, particularly from studies of outbred species. With the advent of microarrays, high-density multiple trait gene expression datasets are now prevalent. Paralleled with the availability of dense SNP marker maps for thousands of individuals this results in a computationally intensive and complex QTL mapping problem. The GridQTL project aims to provide an expanded and improved QTL analysis tool that harnesses Grid technologies to deal with greatly increased computational demands.