BACKGROUND AND OBJECTIVE: To evaluate the repeatability, reliability, and comparability of macular thickness measurements between three optical coherence tomography (OCT) machines in healthy eyes, eyes with diabetic macular edema (DME), and eyes with neovascular age-related macular degeneration (nAMD). PATIENTS AND METHODS: Twenty-three eyes with DME, 26 eyes with nAMD, and 24 healthy eyes as controls were evaluated. Scans were performed using the swept-source Triton (Topcon, Tokyo, Japan), the spectral-domain Cirrus (Carl Zeiss Meditec, Dublin, CA), and the Spectralis (Heidelberg Engineering, Heidelberg, Germany) machines. Scans were evaluated for central macular thickness (CMT), presence of segmentation and fixation imaging artifacts (IA), re-scan reliability, and agreement between machines and groups. RESULTS: Mean CMT was significantly different between all OCT machines in all groups (P < .01 for all comparisons). Manually correcting IA did not alter these results. There was good scan repeatability among healthy and DME eyes for each machine, but poor repeatability among the nAMD group with the Spectralis (P = .038). IA were significantly increased in the presence of pathology. CONCLUSIONS: There is poor agreement of CMT measurement between OCT machines in healthy eyes and those with DME and nAMD. DME and nAMD have a significant effect on the rate of IA in scans. Care is required when interpreting measurements from different OCT devices in clinical practice and research settings.