Three novel mutations, Q54P, W70X and T1081, were identified in the gene encoding glucose-6-phosphatase in three patients with glycogen storage disease type Ia. Two sibs of Portuguese origin were homozygous for the Q54P mutation whereas the third patient, originating from both France and Lebanon, was a compound heterozygote for the W70X and T108I mutations. Glycogen storage disease type Ia is a heterogeneous autosomal recessive condition.