The present study examined the relationship between polymerase chain reaction (PCR) test positivity and clinical outcomes of vitamin D levels measured within the 6 months before the PCR test in coronavirus disease 2019 (COVID-19)-positive patients. In this retrospective cohort study, COVID-19 (227) and non-COVID-19 patients (260) were divided into four groups according to their vitamin D levels: Group I (0-10 ng/ml), Group II (10-20 ng/ml), Group III (20-30 ng/ml), and Group IV (vitamin D > 30 ng/ml). Laboratory test results and the radiological findings were evaluated. In addition, for comparative purposes, medical records of 1200 patients who had a hospital visit in the November 1, 2019-November 1, 2020 period for complaints due to reasons not related to COVID-19 were investigated for the availability of vitamin D measurements. This search yielded 260 patients with tested vitamin D levels. Vitamin D levels were below 30 ng/ml in 94.27% of 227 COVID-19-positive patients (average age, 46.32 ± 1.24 years [range, 20-80 years] and 56.54% women) while 93.07% of 260 non-COVID-19 patients (average age, 44.63 ± 1.30 years [range, 18-75 years] and 59.50% women) had vitamin D levels below 30 ng/ml. Nevertheless, very severe vitamin D deficiency (<10 ng/ml) was considerably more common in COVID-19 patients (44%) (average age, 44.15 ± 1.89 years [range, 23-80 years] and 57.57% women) than in non-COVID-19 ones (31%) (average age, 46.50 ± 2.21 years [range, 20-75 years] and 62.5% women). Among COVID-19-positive patients, the group with vitamin D levels of  >30 ng/ml had significantly lower D-dimer and C-reactive protein (CRP) levels, number levels, number of affected lung segments and shorter hospital stays. No difference was found among the groups in terms of age and gender distribution. Elevated vitamin D levels could decrease COVID-19 PCR positivity, D-dime and CRP levels and the number of affected lung segments in COVID-19-positive patients, thereby shortening the duration of hospital stays and alleviating the intensity of COVID-19.