The association between red blood cell hypervolaemia (RCHV) and racing performance was examined in 116 Standardbred trotters referred to the clinic during a 5 year period with histories of impaired racing performance. Red cell hypervolaemia was the only significant abnormality detected in a thorough clinical examination, cell volumes ranging 116-178% of the predicted normal values. They were also compared with the performance traits of the contemporary Swedish Standardbred trotter population (n = 58,058). The performance traits were based on earnings per year and start, best annual racing time, % placings 1 to 3, and calculated individual performance indices (PI) published in the Annual Statistics for Swedish Trotting. These parameters were transformed to approach normal distributions. The results indicated that horses developing RCHV tended to have been superior athletes initially compared to the contemporary population average. After 3 to 4 years of racing (at mean age +/- s.d. 5.5 +/- 1.5 years) their racing performance declined rapidly during the year of diagnosis of RCHV. Therefore, both earning and placing traits were reduced and best racing time increased. Further, the PI, well above the population average prior to diagnosis, was reduced significantly. During the 3 year period following diagnosis, a slight improvement in racing performance was noted although it was not restored to previous racing form. It was concluded that RCHV in the Standardbred trotter may denote the end of a successful racing career.