Genotypes and phenotypes were studied in 31 Turkish HbS-β-thalassemia patients. In 19 patients the β-thalassemia mutations were β+ and in 12 the β° phenotype. The IVSI-110 mutation was found in 45% of the patients. IVSI-1, β39, IVSII-1 and FSC8 are the genotypes associated with β°-thalassemia. Hematological data were evaluated at the time of diagnosis and 4 years after diagnosis. The mean HbF value was 13 ± 7.8% at diagnosis and 9.7 ± 7.8% 4 years later. A significant negative correlation was observed between the age of the patients and the HbF value (p < 0.05). No statistically significant differences were observed between the mean of hematological parameters in β+- and β°-thalassemia patients except for the mean HbF value which were 10.7 ± 6.9 and 15.9 ± 7.7% in β+- and β°-thalassemia, respectively (p < 0.05). The study indicated that β-thalassemia mutations in trans to the HbS mutation do not exert any beneficial effect on the manifestation of the disease.