Abstract: Sport horse studbooks frequently use performance tests to identify, evaluate and approve stallions for future breeding programmes. The aim of this study was to analyse behaviour-related traits of Polish Warmblood stallions participating in studbook 100-day performance tests. Both relatedness and differences of traits and their contribution to overall variability were analysed. Three-year-old stallions (n = 374) participated in 100-day performance test programmes at two Polish training stations from 2004 to 2008. Traits including character, temperament and trainability (trainer rated), free-jumping, jumping under the rider, walk, trot and canter (independently rated by both trainer and the selection jury) and jumping ability, dressage-ability and rideability (rated by the independent test rider) were subjected to Factor Analysis (FA) with varimax rotation. The FA produced three factors [Cooperation (Cp), Jumping Potential (JP) and Dressage Potential (DP)] with eigenvalues > 1, which accounted for 64.6% of the total variance of the traits examined. After rotation, the factors represented Cp: 23.2.0%, JP: 21.5% and DP: 19.8% of variance respectively. The traits of character, temperament, trainability, jumping under the rider, rideability, dressage and jumping abilities had high loadings on the second factor (Cp) and thereafter, the assessment of all behaviour-related traits contributed to the general evaluation with approximately one-fifth of total variability. The findings may indicate that successful sport careers for any horse may be much more closely associated with willingness to cooperate with the rider and riding and handling behaviour, than to the horse's actual physical performance potential. It would appear that only so-called sport-suitable performance of the horse is being selected for in performance tests (during jumping and dressage suitability testing). This approach may be short-sighted and inadequate with serious implications for horse under-performance in leisure activities notwithstanding general horse welfare within the disciplines of equitation.