In the present study we report on evolution of calicivirus RNA from a patient with chronic diarrhea (i.e., lasting >2 years) and viral shedding. Partial sequencing of open reading frame 1 (ORF1) from 12 consecutive isolates revealed shedding of a genogroup II virus with relatively few nucleotide changes during a 1-year period. The entire capsid gene (ORF2) was also sequenced from the same isolates and found to contain 1,647 nucleotides encoding a protein of 548 amino acids with similarities to the Arg320 and Mx strains. Comparative sequence analysis of ORF2 revealed 32 amino acid changes during the year. It was notable that the vast majority of the cumulative amino acid changes (8 of 11) appeared within residues 279 to 405 located within the hypervariable domain (P2) of the capsid protein and hence were subject to immune pressure. An interesting and novel observation was that the accumulated amino acid changes in the P2 domain resulted in predicted structural changes, including disappearance of a helix structure, and thus a possible emergence of a new phenotype. FUT2 gene polymorphism characterization revealed that the patient is heterozygous at nucleotide 428 and thus Secretor+, a finding in accordance with the hypothesis of FUT2 gene polymorphism and calicivirus susceptibility. To our knowledge, this is the first report of RNA evolution of calicivirus in a single individual, and our data suggest an immunity-driven mechanism for viral evolution. We also report on chronic virus excretion, immunoglobulin treatment, and modification of clinical symptoms, our observations from these studies, together with the FUT2 gene characterization, may lead to a better understanding of calicivirus pathogenesis.