: OBJECTIVES: The purpose of this study was to examine the relationships between climatic and immune or hematologic variables in the peripheral blood of normal human. METHODS: Twenty-six normal volunteers gave blood samples monthly during one calendar year for flow cytometric assays of peripheral blood mononuclear cells (PBMC) and assays of red blood cell (RBC)- and platelet-related variables. Time relationships between the weather and immune or hematologic variables were investigated by means of multiple regression and bivariate cosinor analyses. RESULTS: Highly significant relationships were found among number and percentage of neutrophils, lymphocytes, CD4+, CD8+, CD25+ (interleukin-2-receptor bearing lymphocytes), CD20+ B lymphocytes, number of platelets and RBC, hemoglobin (Hb), mean corpuscular (MC) volume, MC Hb, MC Hb concentration, mean platelet volume or plasma fibrinogen levels and ambient temperature, sunlight duration, air pressure, wind speed, relative humidity, and rainfall duration/day. An important part of the variability in the immune and hematologic variables could be explained by the composite effects of contemporaneous and lagged climatic variables. Common seasonal rhythms were detected in the time series of the above immune/hematologic and sun insolation variables, such as ambient temperature. CONCLUSIONS: The results suggest that i) short-term fluctuations in atmospheric activity modulate immune and hematologic features in the peripheral blood of normal human; and ii) the seasonal rhythms observed in immune/hematologic variables may be entrained by the seasonal rhythms in ambient temperature.