A longitudinal study on the naturally acquired humoral immune responses against the merozoite surface protein 1 of Plasmodium vivax (PvMSP-1) was performed in malaria patients from the Brazilian Amazon region of Rondonia. We have previously cloned and expressed a recombinant protein, ICB2-5, that encodes 508 amino acids from the N-terminal portion of the PvMSP-1 protein. This affinity-purified polypeptide was tested by an enzyme-linked immunosorbent assay in a one-year longitudinal study using sera from 34 patients who had at least one malaria infection during the study period. The results demonstrated that more than 90% of the sera from patients having experienced more than three previous malaria infections contained antibodies to ICB2-5 at the time of a new clinical episode. Unexpectedly, more than half of these multiple-infected patients had an antibody response to ICB2-5 in which the predominant isotype was IgM. In contrast, more than 83% of the sera from these same patients contained predominantly IgG antibodies against total blood-stage antigen preparations. To determine if these results were due to the lack of boosting against this portion of the PvMSP-1 molecule, the presence of IgG antibodies to ICB2-5 in the sera from 11 patients who had consecutive malarial episodes during the study year was investigated. Five of these eleven patients failed to produce IgG antibodies to ICB2-5 even after 1–3 infections. Thus, these results suggest that no boosting against this region of the PvMSP-1 molecule was achieved by natural infections among these patients.