This study examined the relationship of avian reovirus spread and replication to induction of lesions and the relevant role of the S1 segment encoding a virus-neutralizing antigen. One-day-old broiler chickens were infected via footpad or orally with two virus strains (883 and 176) that differ greatly in virulence and a reassortant (R44) that has the S1 segment from 176 and the remaining genome segments from 883. Virus replication and histological lesions in various tissues (heart, liver, spleen, kidney, bursa, hock joint, and bone marrow) were measured at 2-day intervals until day 8 postinoculation. The virulent strain 176 spread to and replicated efficiently in all tissues examined and caused extensive and severe lesions, whereas the mild strain 883 was detected only in tissues near inoculation sites and caused only minimal lesions. The appearance of lesions correlated with the presence of viral replication in each tissue tested. Together, these results indicate that induction of lesions, or pathogenicity, is directly related to virus spread and replication. Reassortant R44 behaved like strain 176 in chicken embryo fibroblasts (CEFs), i.e., both replicated much faster and produced larger plaques than strain 883. In broiler chickens, however, R44 behaved like strain 883, replicating and inducing lesions to an extent that was fat lower than that of strain 176. These results suggest that the S1 segment alone is capable of determining viral replication and plaque formation in cultured CEFs but is not sufficient to determine the virus spread and replication and the pathological change in broiler chickens.