Objective: To explore the risk factors and prophylaxis and treatment of cytomegalovirus interstitial pneumonitis(CMV-IP) after allogeneic peripheral blood stem cell transplantation (allo-PBSCT). Methods: 43 patients who received allo-PBSCT were allocated to either a Gancyclovir(GCV)-prophylaxis group (n=19) or a non-GCV prophylaxis group (n=24).A comparison was made of the incidence of CMV-IP in patients given or not given prophylactic gancyclovir. Results: 9patients in non-GCV prophylaxis group developed late CMV-IP (P＜0.05). Graft-versus-host-disease (GVHD) may be associated with a high risk of CMV-IP. 5 cases of CMV-IP were successfully treated with GCV, but 3 cases died of CMV-IP.The most common adverse event of GCV was neutropenia, but was reversible. Conclusion: CMV infection was a major cause of interstitial pneumonitis after allo-PBSCT, which correlated strongly with the severity of GVHD. Gancyclovir was shown to be effective in both prophylaxis and treatment of CMV-IP.