ObjectivesThe aim of this study was to evaluate the utility of ultrasonographic measurement of the diameter of the inferior vena cava (IVCD) and abdominal aorta (AAD) for assessing volume status.Material and methodsThis was a prospective, observational study. A total of 23 volunteers participated in the study. Each participant was selected randomly. All participants completed the 2016 Kaunas Marathon. Participants filed out a brief survey about their fluid intake (in standardised glasses) in the 24 h before the race and during the race. Participants underwent ultrasound measurements 10–40 min before the start of the race and 3–15 min after finishing the race. To visualize respiratory variation, M-mode was used, with the beam crossing the IVCD 2 cm from the right atrium. The AAD was measured 1 cm above the celiac trunk. IVCD in expiration (IVCDexp)/AAD was calculated by dividing the value of IVCDexp by the value of AAD. The findings were compared with difference in body mass index.ResultsThe mean weight lost after the marathon was 2.93 kg (p < 0.001). Mean IVCD in inspiration (IVCDins) after the run was lower by 0.39 cm (p < 0.001) then before the run. Mean IVCDexp/AAD after the run was 0.24 cm lower than before the run (p = 0.03). Before and after the marathon, there was a statistically significant negative correlation in weight difference, with mean IVCDexp difference (p = 0.047). There was no statistically significant difference in caval index before and after running.ConclusionUltrasonographic assessment of IVCDexp could be useful in the evaluation of volume status.