Recurrent respiratory tract infections in children are a common health problem. Exposure to harmful environmental agents and coexistence of chronic diseases affect the severity and rate of infections. Ineffective outpatient treatment is an indication for a more comprehensive diagnostic workup. The aim of the study was to determine the cause of recurrent respiratory tract infections in children. The study group included 130 children aged 3–17 years who were referred for diagnostic investigation to determine the cause of recurrent respiratory tract infections. The eligibility criterion for the study group was the occurrence of 8 or more respiratory tract infections a year in children aged up to 6 years and 5 or more infections a year in children aged 7–17 years. The study group was subdivided into the following age groups: 3–5 years (n = 60), 6–9 years (n = 35), 10–13 years (n = 11) and 14–17 years (n = 24). The presence of potential risk factors for recurrent infection was analysed on the basis of questionnaires. Tests and examinations were also performed in order to exclude allergies, ear, nose and throat disorders and gastroesophageal reflux disease. Selected laboratory values (complete blood count, vitamin D concentration, immunoglobulin levels) were compared with those obtained from 86 healthy children from the control group. The majority of children had risk factors for recurrent respiratory tract infections. Adenoid hypertrophy was diagnosed in 44.6% of children, asthma in 36.9%, vitamin D deficiency in 30.7%, gastroesophageal reflux disease in 16.2% and immunodeficiency in 9.2% of children. Multimorbidity was found in 34.6% of the subjects. In the study group, there were higher vitamin D levels [study group: 25.6 ng/mL (25th percentile = 17; 75th percentile = 33.3), control group: 22.84 ng/mL (25th percentile = 16.3; 75th percentile = 28.7); p < 0.044] and higher leukocyte levels [study group: 7.5 × 103/μL (25th percentile = 5.9; 75th percentile = 9.1), control group: 6.4 × 103/μL (25th percentile = 5.3; 75th percentile = 7.7); p < 0.02]. The results indicate that children with recurrent respiratory tract infections are a heterogeneous group of patients. The youngest children were the largest group (n = 60). In the majority of children, the onset of symptoms was associated with the beginning of nursery/preschool or school attendance. In 1 in 3 children, a combined presence of a few diseases was found which increase susceptibility to recurrent respiratory tract infections.