Obstructive sleep apnea (OSA) is associated with increased carotid intima–media thickness (CIMT), which is an early marker of atherosclerosis. Continuous positive airway pressure (CPAP) is the first-line of treatment for OSA. A prospective study was performed to determine whether CPAP therapy could decrease CIMT. Objective To estimate the effect of CPAP treatment and also the degree of OSA severity on CIMT in patients with OSA. Patients and methods A prospective study was conducted on 50 patients who were newly diagnosed with OSA. The patients were divided into two groups: the CPAP group who were compliant on using CPAP device and the conservative treatment (CT) group who refused to use the CPAP device and remained only on conservative measures. CIMT was measured (with B-mode ultrasound) for both groups at the start of the study and after 6 months. Result We have 27 and 23 OSA patients who received the CPAP device and CT, respectively. We found no significant differences between both groups regarding the baseline parameters. Also at the start of the study there were no significant differences regarding the polysomnographic (PSG) parameters and ultrasonographic measurements of CIMT. CIMT was found to be high in OSA patients. After 6 months of study, there were significant improvement in PSG parameters and a significant reduction in CIMT in the CPAP group while in the CT group the CIMT increased and the PSG parameters became worse. Conclusion Patients with OSA had an increased CIMT suggestive of an atherosclerotic process. The thickness was higher in severe OSA. CPAP treatment was found to decrease CIMT significantly in OSA patients with CPAP compliance