Although it is known that red blood cell (RBC) parameters and platelet count depend on ethnicity and sex, their reference intervals in healthy Asian populations are limited. The aim of this study was to establish reference intervals for RBC parameters and platelet count for healthy adults in Japan. A total of 750 healthy adults (447 women and 303 men; median age 40 years (18-67 years) at seven Japanese centers who participated in regular medical checkups entered this study. Their RBC parameters and platelet count were measured using automated hematocytometers. The reference intervals of the RBC parameters and platelet count according to sex in healthy adults were determined. There was an age-specific decrease in RBC counts and an age-specific increase in mean corpuscular volume in men. This study emphasizes the need to consider sex and age in the clinical use of reference intervals of RBC parameters.