Objectives: To evaluate the clinical manifestations, course, and outcomes of COVID-19 amongst patients with RAD and DM and compare these two groups concerning SARS-CoV2 infection. Materials and methods: A cross-sectional study was done in the Department of Rheumatology and Clinical Immunology and Clinic of Infectiology medicine of the University Hospital Split (KBC Split) of the University of Split. Of the 148 patients selected, 74 were diagnosed with rheumatologic disease, and 74 were diagnosed with diabetes mellitus. The EULAR-COVID-19 Survey Checklist categorized the medical data of eligible patients. The following variables were included: Age, sex, diagnosis, symptoms, treatment, complications, outcome, laboratory parameters (CRP). Results: Patients with DM were statistically significantly more inpatients comparing to patients with RAD (P<0.01). There was a statistically significant difference between the studied groups regarding the test methods used (P<0.01). Rheumatologic patients were treated to a lesser extent for COVID-19 than patients with diabetes, where a combination of treatments was used the most (P<0.01). Rheumatologic patients had statistically fewer respiratory COVID-19 complications than patients with diabetes mellitus (P<0.01). There was a statistically significant difference in the disease outcome of COVID-19 between the two studied groups. Patients with diabetes mellitus had a worse outcome than patients with rheumatologic disease, which was statistically significant (P<0.01). The patients from the RAD group had lower CRP values that averaged 3.60 with a 95% CI of 1.93 to 5.02. In contrast, the patients from the DM group had higher CRP values with a median of 83.1 and a 95% CI of 44.78 to 125. 73. Conclusion: Our results showed that patients with rheumatic autoimmune disease tend to have a better disease outcome than patients with diabetes mellitus. Patients with diabetes mellitus had more complications, mainly respiratory, and needed more extensive treatment than the compared RAD group.