We have reported that attenuation of the virulence of a field Sendai virus (SeV) isolated by egg passage is associated with an impediment of viral genome replication in mouse respiratory cells (Kiyotani et al., Arch Virol 146, 893-908, 2001). To determine the molecular basis for the attenuation, we sequenced entire genomes of representative SeV clones isolated during egg passages and compared those with that of the parental SeV clone E0. E15cl2, a 165-fold attenuated clone in 50% mouse lethal dose (MLD50) isolated at the 15th egg passage, possessed only four mutations in the entire genome: U to A at position 20 (U20A) and U24A in the leader promoter region and A9362G and A12174U in the L gene from the 5'-end of antigenome. The former mutation in the L gene was silent and the latter changed deduced amino acid Ser at position 1207 to Cys (Ser1207Cys) in the L protein, a catalytic subunit of viral polymerase. E30cl2, a further 6-fold attenuated clone isolated at the 30th egg passage, had an additional four mutations: A8074G (Glu461Gly) and A8077G (Asp462Gly) in the hemagglutinin-neuraminidase (HN) gene and A13598C (silent) and G13927A (Ser1791Asn) in the L gene. On the other hand, a virulent revertant clone, E30M15cl5, which was obtained by 15 mouse passages of E30cl2 and had 250-fold mouse virulence compared to E30cl2, possessed eight mutaions: A24U in the leader, C1325U (silent) in the nucleocapsid gene, G8074A (Gly461Glu) in the HN gene, G10433U (Lys626Asn), C13598A (silent), A13927G (Asn1791Ser), C14626U (Thr2024Ile) and A15272C in the L gene. Among these, the mutations in the leader and the HN gene and two of the mutations in the L gene (C13598A and A13927G) were true reversions to E0. The significance of the mutations detected in the leader as well as in the L and HN genes was discussed in the context of attenuation of SeV pathogenicity by egg passage.