Objective: This study aimed to define the predictors of critical illness development within 28 days postadmission during the first wave of the COVID-19 pandemic. Materials and Methods: We conducted a prospective cohort study including 477 PCR-positive COVID-19 patients admitted to a tertiary care hospital in Istanbul from March 12 to May 12, 2020. Results: The most common presenting symptoms were cough, dyspnea, and fatigue. Critical illness developed in 45 (9.4%; 95% CI=7.0%-12.4%) patients. In the multivariable analysis, age (hazard ratio (HR)=1.05, p <0.001), number of comorbidities (HR=1.33, p=0.02), procalcitonin ≥0.25 μg/L (HR=2.12, p=0.03) and lactate dehydrogenase (LDH) ≥350 U/L (HR=2.04, p=0.03) were independently associated with critical illness development. The World Health Organization (WHO) ordinal scale for clinical improvement on admission was the strongest predictor of critical illness (HR=4.15, p<0.001). The patients hospitalized at the end of the study period had a much better prognosis compared to the patients hospitalized at the beginning (HR=0.14; p=0.02). The C-index of the model was 0.92. Conclusion: Age, comorbidity number, the WHO scale, LDH, and procalcitonin were inde- pendently associated with critical illness development. Mortality from COVID-19 seemed to be decreasing as the first wave of the pandemic advanced.