BACKGROUND: The immunologial alterations of tertian malaria are poorly understood. We investigated the hematological and immunological findings to know immunological mechanism of tertian malaria. METHODS: Forty patients with tertian malaria, hospitalized in the three affiliated hospitals of Catholic University Medical College, were enrolled in this study. The hematologic examination was performed by Coulter STKS. Atypical lymphocytes, eosinophils and plasmodium burden were counted manually. The immunoglobulin and complement concentrations were measured by nephelometry( Behring nephelometer analyzer, Germany) and automated chemiluminescence system(ACS 180, USA). The peripheral blood lymphocyte subsets were analyzed by flow cytometry using anti- CD3, CD4, CD8, CD19, CD25 and CD56 monoclonal antibodies(Becton Dickinson, San Jose, USA) and negative control. RESULTS: Anemia, lymphopenia, thrombocytopenia and eosinopenia were observed, and the eosinophil count was correlated to platelet count. The numbers of CD3+, CD8+, CD19+, CD56+, CD3-/CD56+ and CD8+/CD56+ lymphocytes were lower in tertian malaria than in control group(P<0.05). At 8th week after treatment, the percentage of CD8+ lymphocytes became significantly higher than before. The percentage of CD19+ lymphocytes was correlated to the number of eosinophils and thrombocytes(r=0.641, 0.417, P=0.000, 0.006). The serum concentrations of IgM and IgE were higher in tertian malaria than in control group. At 1st week after treatment, the IgE concentration became significantly lower than acute stage(P=0.014). The C3 and C4 concentrations were higher in tertian malaria than in control group. The C4 concentration became the same to the control group at the first week after treatment and was correlated to hemoglobin concentration. CONCLUSIONS: The eosinopenia with high IgE concentration could be a valuable marker of tertian malaria and IgE and C4 concentrations could be useful for serial monitoring after treatment.