Parainfluenza virus type 1 (PIV1) is a major cause of croup in infants and young children, and a vaccine is needed to prevent the serious disease caused by this virus. In the present study, a live attenuated PIV1 vaccine candidate was generated by modification of the extensively-studied PIV3 cold-passaged (cp) cp45 vaccine candidate using the techniques of reverse genetics. The HN and F glycoproteins of the PIV3 cp45 candidate vaccine virus were replaced with those of PIV1. This created a live attenuated PIV1 vaccine candidate, termed rPIV3-1 cp45, which contained the attenuated background of the PIV3 cp45 vaccine virus together with the HN and F protective antigens of PIV1. Three of the 15 mutations of cp45 lie within the HN and F genes, and those in the F gene are attenuating. Thus, some attenuation might be lost by the HN and F glycoprotein replacement. To address this issue we also constructed a derivative of PIV3 cp45, designated rPIV3 cp45 (F_wtHN_wt), that possessed wild type PIV3 HN and F glycoproteins but retained the 12 other cp45 mutations. rPIV3 cp45 (F_wtHN_wt) replicated in the respiratory tract of hamsters to a level three- to four-fold higher than rPIV3 cp45, indicating that loss of the two attenuating mutations in the cp45 F gene effected a slight reduction in the overall attenuation of cp45 for hamsters. However, the chimeric rPIV3-1 cp45 virus was about 5-fold more restricted in replication in hamsters than rPIV3 cp45 and about 15- to 20-fold more restricted than rPIV3 cp45 (F_wtHN_wt). This suggests that two components contribute to the attenuation of the new chimeric rPIV3-1 cp45 PIV1 vaccine candidate: one being the 12 cp45 mutations, which provide most of the observed attenuation, and the other resulting from the introduction of the heterologous PIV1 HN and F proteins into PIV3 (i.e., a chimerization effect). rPIV3-1 cp45 was observed to be immunogenic and protective against challenge with wild type PIV1 in hamsters. This virus shows sufficient promise that it should be evaluated further as a candidate live attenuated vaccine strain for preventing severe lower respiratory tract PIV1 disease in infants and young children.