Increased expression of fetal haemoglobin (HbF) may ameliorate the clinical course of beta-thalassemia and sickle cell disease. Some pharmacological agents, such as hydroxycarbamide (HC), can increase fetal haemoglobin synthesis during adult life. Cellular selection and/or molecular mechanisms have been proposed to account for this increase. To explore the mechanism of action of HC we focused on homozygous Hb-Lepore patients that presented with high fetal haemoglobin levels and were good responders to HC treatment "in vivo". We performed primary erythroid cultures from peripheral blood of four homozygous Lepore patients. The increase in HBG (gamma-globin) transcription levels and HbF content in these cultures, after HC treatment, were detected by quantitative real time polymerase chain reaction analysis and flow cytometric analysis. Primary transcript "in-situ" hybridization analysis showed a 2-fold increase in the number of cells expressing both HBG alleles in HC-treated erythroid cultures. These studies, demonstrating the larger number of biallelic HBG expressing cells, suggest that HC is able to stimulate the activation of HBG transcription. These observations provide evidences that the molecular mechanism of action is involved in the increase of fetal haemoglobin production by HC.