This paper describes the first human gene therapy trial for hemophilia B. Retroviruses were used to introduce human factor IX into autologous, primary human skin fibroblasts from the patients. Recombinant retroviral vector containing human FIX cDNA driven by viral LTR promoter (XL-IX) and double-copy retroviral vector driven by human cytomegalovirus enhancer-promoter (N2CMV-IX) were constructed. After the safety assessment, including soft-agar test, cell morphology observation, analysis of endotoxin, chromosome karyotype, allergic reaction test, nude mice test, routine pathological test, electromicroscopic analysis, and virus detection by PCR, etc., the engineered cells were pooled and embedded in collagen mixture, autologously injected into the patients respectively. The concentration of human FIX protein of Patient 1 increased from 71 ng/ml to 220 ng/ml, with a maximum level of 245 ng/ml. The expression of FIX has lasted for 6 months at the time of writing. The clotting activity also increased from 2.9% to 6.3%, his clinical symptoms have been alleviated obviously. The secretion rate of FIX for Patient 2 increased from 130 to 250 ng/ml, maintained at the level of 220 ng/ml for 5.5 months at the time of writing, but the clotting activity has not been increased steadily. There is no deleterious effect to be found in the two patients since the ex-vivo cells were implanted. The two patients are now under follow-up investigation. We suggested that retrovirus-mediated transfer of genes into skin fibroblasts, to be embedded in collagen and subcutaneously injected into patients, is a simple and effective approach for the gene therapy for hemophilia B.