INTRODUCTION: The aim of this study investigated the effect of eosinophil and basophil counts on mortality in patients with COVID-19 infection. METHODS: Blood tests of 582 patients with RT-PCR test in an oropharyngeal swab sample who were admitted to Diyarbakır Gazi Yaşargil Training and Research Hospital between March 2020 and December 2020 were retrospectively analysed. The patients were divided into two groups: those who recovered and discharged and those who had a mortal course. Demographic data, comorbid diseases, routine blood tests, and haematological parameters were compared between both groups. RESULTS: An eosinophil count of 0.01 ± 0.04 and basophil count of 0.0261 ± 0.026 was observed in the patient group who had a mortal course at first admission to the hospital, while the eosinophil count was 0.06 ± 0.12 and basophil count was 0.020 ± 0.017 in the recovered patient group. On the fifth day after admission, the eosinophil count was 0.02 ± 0.07 and basophil count was 0.043 ± 0.042 in the patient group with a mortal course, while the count of eosinophils was 0.13 ± 0.14 and basophils was 0.023 ± 0.016 in the recovered patient group. In both groups, the eosinophil and basophil counts on the fifth day increased compared to the first day, and the eosinophil and basophil counts were lower in those who had a mortal course on both the first and fifth day (p < 0.05). CONCLUSIONS: In our study, a significant decrease was observed in the count of eosinophils and basophils in the mortal group of COVID-19 patients. Eosinopenia and basopenia may be parameters that can be used to facilitate the diagnosis of COVID-19, and the depth of both eosinopenia and basopenia is positively correlated with COVID-19 mortality.