PURPOSE: To compare retinal thickness (RT) measurements obtained by spectral-domain optical coherence tomography (SD-OCT) and time-domain OCT (TD-OCT) in eyes with and without posterior segment disease diagnoses. DESIGN: Retrospective, observational chart review. METHODS: Scans of 184 healthy and diseased eyes from 106 patients were included in the study. Pair-wise comparisons of RT measurements from SD-OCT machines from 2 different manufacturers and TD-OCT were performed for all scans and with eyes stratified by disease diagnosis. RESULTS: Foveal thickness measurements obtained from both SD-OCT machines were greater than those measured by TD-OCT, by 51.0 +/- 23.8 microm and 72.5 +/- 30.4 microm, respectively. The foveal thickness difference between the 2 SD-OCT models was 24.3 +/- 19.1 microm. Foveal thickness differences varied by disease diagnosis. CONCLUSIONS: RT measurements obtained by SD-OCT are consistently greater than those obtained by TD-OCT. RT measurement differences may vary by disease diagnosis or by SD-OCT model. Consideration of these measurement differences is essential when OCT-determined RT measurement data are used in clinical settings.