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Hemoglobin concentrations and red blood cell counts were repeatedly determined once every four seasons of the year in 324 apparently healthy adults (181 men and 143 women) in 15 groups at 7 locations (from the northern-most at Sapporo to the southern-most at Bangkok) in 4 Asian countries of different cli­matic conditions. Work intensity of the subjects was essentially constant through­out the year. Both hematological variables were significantly lower in summer than in winter in the areas where seasonal variation in the outdoor temperature was pronounced, i.e., higher than 20°C in summer and lower than 5°C in winter. There was essentially no seasonal change in hemoglobin concentration or red blood cell counts where the temperature stays between 25 and 30°C throughout the year. The seasonal variation in hemoglobin concentration and red blood cell counts appeared to be due to temperature and not to relative humidity. When the work rooms were air-conditioned, no seasonal change in the blood variables of the subjects was observed.