['Longevity/durability is a relevant trait in racehorses. Genetic analysis and knowledge of factors that influence number of harness race starts would be advantageous for both horse welfare and the equine industry.', 'To perform a genetic analysis on harness racing using number of races as a measure of longevity/durability and to identify factors associated with career length in Spanish Trotter Horses (STH).', 'Longitudinal study.', "Performance data (n = 331,970) on the STH population for harness racing at national level between 1990 and 2014 were used. A grouped data model was fitted to assess factors influencing the risk of ending harness racing career and to estimate the heritability and breeding values for total number of harness races starts as an indicator of horses' longevity and durability. The model included sex, age at first race and first start earnings as time-independent effects, and the calendar year, driver, trainer, racetrack category and season of competition as time-dependent effects.", "Across the whole dataset, the average number of harness races horses achieved in Spain was 54.7 races, and this was associated with the horses' sex, age at first race and first start earnings, calendar year, driver, racetrack category, and season. The heritability estimated (0.17 ± 0.01) for number of harness race starts indicates that a beneficial response to direct genetic selection can be expected.", "Data on horses' health status were not available."] Does heritability and factors associated with number of harness race start in the Spanish Trotter horse population?