objectives: this thesis is aimed to detect the incidence of mood disorders in cancer patients, and to assess the role of arthritis and multimorbidity in the development of mood disorders. the thesis will also describe the impact of arthritis and mood disorders on the health care utilization of cancer patients accounting for the role of other comorbidities. methods: the thesis is a population-based study with linked administrative health databases, available from the institute for clinical evaluative sciences, and included all adult patients diagnosed with cancer in ontario between 2003 and 2013. the study used a retrospective cohort design. univariate and bivariate analyses were used to describe the incidence of mood disorders, level of multimorbidity, and characteristics of patients with cancer. survival analysis was used to evaluate the adjusted impact of arthritis and other variables on the occurrence and time to the development of mood disorders. negative binomial regression was performed to assess the adjusted effect of arthritis and mood disorders on health service utilization by cancer patients. results: the cumulative crude incidence of mood disorders over the study period in cancer patients in ontario was 16.9 (95% ci 16.6-17.3)/1,000 population. arthritis was not associated with an increased risk of developing mood disorders in crude and adjusted analysis; however, multimorbidity significantly increased the risk of mood disorders. mood disorders and arthritis increased health care utilization in cancer patients. the average hospitalizations per person-year in patients with mood disorders diagnosed before (0.73± 1.06 ) and after (1.09 ± 1.33) cancer was higher than patients without mood disorders (0.57 ± 0.92). similarly, average ed visits per personyear were also higher when mood disorders were diagnosed before (1.03 ± 1.68 ) or after (1.20 ± 2.10 ) cancer compared to the patients without mood disorders. arthritis increased only ed visits while decreasing hospitalization. when mood disorders and arthritis were both present in patients with cancer and diagnosed after cancer there was a 28% (95% ci = 20-37%) higher risk of hospitalization and a 60% (95% ci = 50-69%) increased risk of ed visits compared to those without any of these conditions. conclusion: the study provided evidence on the risk of developing mood disorders in cancer patients with multiple chronic conditions. multimorbidity is an important risk factor for developing mood disorders and the number of chronic conditions rather than a specific condition like arthritis increases the likelihood of developing mood disorders. the study showed the importance of addressing multimorbidity to improve the mental health outcomes in cancer patients. the findings of the study also described the impact of arthritis and mood disorders on the health service utilization of cancer patients. mood disorders increased health services use more than arthritis. early identification of psychological and other chronic conditions may decrease hospital admissions and ed visits in cancer patients reducing the burden and cost of the health care system.