Objectives: To investigate perceptions and expectations of patients regarding hospital outpatient services by using a service quality gap model and factors influencing such gaps. Methods: In this cross-sectional descriptive study conducted between October and November 2014 in the outpatient waiting areas of a hospital in the Eastern Province of Saudi Arabia, a sample of 306 patients was selected by convenience sampling technique. The data was collected through an Arabic version of the service quality (SERVQUAL) questionnaire consisting of 2 parts: patients' demographic characteristics, and 22 items scales of patients' expectations and perceptions of SERVQUAL. The data was analyzed by confirmatory factor analysis, independent, and paired t samples tests and one way analysis of variance test. Results: The results showed that the proposed model for service quality dimensions had a good fit by satisfying the recommended values. The patients' expectations exceeded perceptions in all service quality dimensions indicating statistically significant service quality gaps (t=26.3, p<0.000). Findings revealed that the empathy dimension contributed most patients' expectations (4.7 +/- 0.5) and perceptions (3.7 +/- 0.8) scores, and responsiveness contributed least to expectations (4.5 +/- 0.6) and perceptions (3.2 +/- 0.8) scores. Prompt services showed highest service quality gap, while observation of privacy showed the smallest service quality gap in the statements. The study showed a significant association between gender, age, education, multiple visits, and service quality dimensions. Conclusion: The proposed model is valid and reliable and significant service quality gaps of all 5 dimensions need to be prioritized and addressed by focused improvement efforts of hospital management.