BackgroundIntravascular volume assessment in foals is challenging. In humans, intravascular volume status is estimated by the caudal vena cava (CVC) collapsibility index (CVC‐CI) defined as (CVC diameter at maximum expiration [CVC max] – CVC diameter at minimal inspiration [CVC min])/CVC max × 100%.Hypothesis/ObjectivesTo determine whether the CVC could be sonographically measured in healthy foals, determine differences in CVC max and CVC min, and calculate inter‐ and intrarater variability between 2 examiners. We hypothesized that the CVC could be measured sonographically at the subxiphoid view and that there would be a difference between CVC max and CVC min values.AnimalsSixty privately owned foals <1‐month‐old.MethodsProspective study. A longitudinal subxiphoid sonographic window in standing foals was used. The CVC max and CVC min were analyzed by a linear mixed effect model. Inter‐rater agreement and intrarater variability were expressed by Bland‐Altman and intraclass correlation coefficients, respectively.ResultsMeasurements were attained from 58 of 60 foals with mean age of 15 ± 7.9 days and mean weight of 75.7 ± 17.7 kg. The CVC max was significantly different from CVC min (D = 0.515, SE = 0.031, P < 0.001). Inter‐rater agreement of the CVC‐CI differed by an average of −0.9% (95% limits of agreement, −12.5 to +10.7%). Intrarater variability of CVC max was 0.540 and 0.545, of CVC min was 0.550 and 0.594, and of CVC‐CI was 0.894 and 0.853 for observers 1 and 2, respectively.Conclusions and Clinical ImportanceThese results indicate it is possible to reliably measure the CVC sonographically in healthy foals, and the CVC‐CI may prove useful in assessing the intravascular volume status in hypovolemic foals.