PURPOSE: To assess the relationship between ciliary body thickness and refractive error. DESIGN: Prospective observational case series. METHODS: We enrolled 75 eyes of 75 patients with no history of intraocular surgery or use of anti-glaucoma medication. Axial length was measured by A-scan ultrasonography. Ciliary body thickness (CBT) was measured by two masked observers on radial-section images of the temporal corneoscleral limbus, acquired with ultrasound biomicroscopy (UBM) at 2 and 3 mm posterior to the scleral spur (CBT2 and CBT3, respectively). RESULTS: Refractive error was inversely correlated with CBT2 and CBT3 (r = -.64 and r = -.52, P < .0001, respectively). Axial length was correlated with CBT (r = .66 and r = .57, P < .0001, for CBT2 and CBT3, respectively). CONCLUSION: CBT increases with increasing myopia and axial length.