A three-year-old female with compound heterozygosity for Hb Knossos and IVS-I-1 mutation is presented. On physical examination she had no abnormality except far pallor. Hb was 6.9 g/dl, MCV 61 fl, Hb A2 2% and Hb F 38.5%. Acrylamidegel electrophoresis at a pH of 6 revealed the presence of Hb Knossos in the child and her father. DNA studies revealed that the child was compound heterozygous for Hb Knossos and the IVS I-1 mutation. When the clinical expression of this combination in a previously reported patient with Hb Knossos/FSC8 mutation is compared, it is shown that the newly presented patient has a more severe condition, indicating that the mutations in the trans of Uh Knossos may play a role in the phenotypical expression of the disease.