BACKGROUND: Data about the morphological changes in peripheral blood smears during COVID-19 infection and their clinical severity association are limited. We aimed to examine the characteristics of the cells detected in the pathological rate and/or appearance and whether these findings are related to the clinical course by evaluating the peripheral blood smear at the time of diagnosis in COVID-19 patients. METHODS: Clinical features, laboratory data, peripheral blood smear of fifty patients diagnosed with COVID-19 by PCR was evaluated at diagnosis. Peripheral smear samples of the patients were compared with the age and sex-matched 30 healthy controls. Pictures were taken from the patient's peripheral blood smear. Patients were divided into two groups. Mild and severe stage patient groups were compared in terms of laboratory data and peripheral smear findings. The relationship between the laboratory values of all patients and the duration of hospitalization was analyzed. RESULTS: The number of segmented neutrophils and eosinophils were low, pseudo-Pelger-Huet, pseudo-Pelger-Huet/mature lymphocyte ratio, atypical lymphocytes, monocytes with vacuoles, bands, and pyknotic neutrophils rates were higher in the peripheral blood smear of the patient group (p <0.05). Increased pseudo-Pelger-Huet anomaly, pseudo-Pelger Huet/mature lymphocyte ratio, a decreased number of mature lymphocytes, and eosinophils in peripheral blood smear were observed in the severe stage patients (p <0.05). A negative correlation was observed between hospitalization duration and mature lymphocyte and monocytes with vacuoles rates (p <0.05). CONCLUSION: A peripheral blood smear is an inexpensive, easily performed, and rapid test. Increased Pseudo-Pelger-Huet anomaly/mature lymphocyte rate suggests a severe stage disease, while high initial mature lymphocyte and monocytes with vacuoles rates at the time of diagnosis may be an indicator of shortened duration of hospitalization.