PURPOSE. To compare retinal vessel calibers extracted from phase-sensitive optical coherence tomography (OCT) images with vessel calibers as obtained from the Retinal Vessel Analyzer (RVA). METHODS. Data from previously published studies in 13 healthy subjects breathing room air (n = 214 vessels) and 7 subjects breathing 100% oxygen (n = 101 vessels) were used. Vessel calibers from OCT phase images were measured vertically along the optical axis by three independent graders. The data from RVA fundus images were corrected for magnification to obtain absolute values. RESULTS. The average vessel diameter as obtained from OCT images during normoxia was lower than from RVA images (83.8 +/- 28.2 mu m versus 86.6 +/- 28.0 mu m, P < 0.001). The same phenomenon was observed during 100% oxygen breathing (OCT: 81.0 +/- 22.4 mu m, RVA: 85.5 +/- 26.0 mu m; P = 0.001). Although the agreement between the two methods was generally high, the difference in individual vessels could be as high as 40%. These differences were neither dependent on absolute vessel size nor preferably found in specific subjects. Interobserver differences between OCT evaluators were much lower than differences between the techniques. CONCLUSIONS. Extracting vessel calibers from OCT phase images may be an attractive approach to overcome some of the problems associated with fundus imaging. The source of differences in vessel caliber between the two methods remains to be investigated. In addition, it remains unclear whether OCT-based vessel caliber measurement is superior to fundus camera-based imaging in risk stratification for systemic or ocular disease.