Background The Hajj is the annual mass gathering of Muslims that occurs in Makkah, Saudi Arabia. The Saudi Vision 2030 predicts the attendance of 30 million pilgrims each year by 2030. Cost-effective healthcare services during the Hajj are important to manage this increase in the number of pilgrims. Communicable diseases have always been a concern during the Hajj. However, little is known about the impact of the preexisting chronic diseases on morbidity and mortality during the Hajj. Furthermore, the quality of services provided by Hajj hospitals warrants further study. Objectives A relatively large number of pilgrims admitted to Hajj hospitals die during their stay. This dissertation aims to describe patterns of inpatient, all-cause mortality during the Hajj and the relationship between mortality and preexisting chronic diseases as well as the services provided in Hajj hospitals. Study Design and Population The population included pilgrims who were admitted to Hajj hospitals in Makkah and sacred sites during five Hajj seasons between 2012 and 2017, excluding 2015. A retrospective, matched, case-control study design was utilized with a ratio of deaths to surviving controls of 1:2, resulting in 2,237 cases of mortality being matched to 4,474 control cases based on age and gender using a one-stage sampling approach. The data was extracted from hospital admissions offices and medical records. Methods Preexisting chronic diseases included diabetes mellitus, hypertension, and cardiovascular diseases. Medical services provided by Hajj hospitals included intensive care unit (ICU) admission, intubation, radiology imaging (MRI and CT scan), endoscopy, and blood transfusion. Covariates included individual-level variables (age, gender, nationality, length of stay, mode of admission, discharge diagnosis, and Hajj status) as well as hospital-level variables (hospital location, hospital of discharge, bed-to-doctor ratio, and bed-to-nurse ratio). Hierarchical, logistic regression models were used to examine the medical services. The effect measure modification of the copresence of more than one chronic disease was also examined. For every independent variable that was evaluated, the prevalence, crude and adjusted odds ratios (AORs), and corresponding 95% confidence intervals (CIs), were calculated. Results The rate of inpatient all-cause mortality was higher in Makkah hospitals compared to sacred site hospitals. Also, inpatient, all-cause mortality was significantly associated with diabetes (AOR: 1.44, 95% CI: 1.27-1.63), hypertension (AOR: 1.34, 95% CI: 1.17-1.53), and cardiovascular diseases (AOR: 1.32, 95% CI: 1.14-1.53). Moreover, effect measure modification was present in the association between diabetes and cardiovascular diseases and the association between hypertension and cardiovascular diseases, but not the association between diabetes and hypertension. Finally, patients who were admitted to the ICU or who received radiology imaging, endoscopy, or blood transfusion were more likely to die during their hospital stay compared to those patients not receiving those services (ICU AOR: 8.00, 95% CI: 7.8-8.2; radiology AOR: 1.60, 95% CI: 0.98-2.60; endoscopy AOR: 1.99, 95% CI: 1.37-2.88; blood transfusion AOR: 4.00, 95% CI: 2.59-6.24). However, the mortality rate was lower in intubated patients compared to nonintubated patients (AOR: 0.54, 95% CI: 0.35-0.82). Conclusion The current focus on public health issues during the Hajj should be equally distributed between communicable and noncommunicable diseases. Although advanced services are provided by Hajj hospitals, interventions to address the increased risks, including mortality, faced by pilgrims with preexisting, chronic diseases should be further investigated and considered.