OBJECTIVE: To evaluate the prognostic value of measuring heart rate variability (HRV) in horses with colic at the time of admission to a referral hospital. ANIMALS: 51 horses > 1 year of age with colic (41 that survived [survivors] and 10 that died or were euthanized [nonsurvivors]). PROCEDURES: HRV was recorded within 1 hour after admission by use of heart rate sensors with horses restrained in stocks. A 5-minute recording period was analyzed to obtain HRV measurements (eg, SD of R-R intervals [SDRR], root mean square of successive differences between R-R intervals [RMSSD], and geometric SDs determined from Poincare plots [SD1 and SD2]). Variables associated with outcome (survival vs nonsurvival) were identified. Measurements were compared among diagnostic categories for colic (obstructive, inflammatory, or ischemic). RESULTS: SDRR and RMSSD were significantly higher in survivors (median [25th to 75th percentile], 91.0 milliseconds [78.9 to 114.6 milliseconds] and 64.8 milliseconds [40.9 to 78.4 milliseconds], respectively) than in nonsurvivors (50.7 milliseconds [29.1 to 69.2 milliseconds] and 33.4 milliseconds [12.6 to 47.9 milliseconds], respectively). Similarly, SD1 and SD2 were significantly higher in survivors (48.3 milliseconds [28.9 to 60.9 milliseconds] and 111.3 milliseconds [93.0 to 146.6 milliseconds], respectively) than in nonsurvivors (23.7 milliseconds [8.9 to 33.9 milliseconds] and 65.1 milliseconds [33.7 to 91.9 milliseconds], respectively). The SDRR and SD2 were significantly higher for horses with obstructive colic than for horses with ischemic colic. CONCLUSIONS AND CLINICAL RELEVANCE: Analysis of HRV in horses with colic may provide information on the underlying cause and be helpful in identifying horses less likely to survive.