Combined immunodeficiency diseases comprise a group of disorders with different molecular basis. Clinical and immunological phenotypes for each group are extremely heterogenous. The frequency of combined immunodeficiencies may vary in different countries. The most frequent forms of combined immunodeficiency show inherited defects in development of T and/or B lymphocytes. These defects are classified according to immunologic phenotype and are categorized into T-B+ or T-B- including forms with or without natural killer lymphocytes. We report here twenty-three patients (female/male: 12/11) with combined immunodeficiency showing different immunological and clinical phenotypes, majority of whom were admitted because of severe upper and lower respiratory tract infections. Mean age of the study group, mean age at onset of the symptoms, and diagnosis were 47.5 +/- 42.2, 11.2 +/- 17.3, and 19.5 +/- 23.8 months, respectively. There was nearly 8 months time delay between beginning of symptoms and diagnosis. Within the combined immunodeficiency phenotypes, T-B-NK+ category was the most frequent phenotype. Consanguinity was positive in 73.9% (n = 17) of patients while it was about 80.0% (n = 8) in deceased ten children. Bone marrow or umblical cord stem cell transplantation was applied to 11 of them. Three patients deceased after transplantation and seven patients deceased without transplantation. Twelve patients are being followed by prophylactic treatment. In conclusion; combined immunodeficiencies are frequent in our country because of high rate of consanguinity. T-B- combined immunodeficiencies are more often observed, and infants presenting severe infections beginning in the first 3 months of life have to be examined for combined immunodeficiencies. Shortening of time delay in diagnosis will increase success of life-saving treatment.