Background To set a reference value for high-sensitivity C-reactive protein (hs-CRP) in a healthy Thai population and study the effect of time, gender and age on that value. Methods and Results Three hundred and sixty-four subjects, aged between 18 and 74 years, comprising 185 men and 279 women, were studied. Another 10 healthy subjects, aged between 18 and 54 years, were recruited for the study of circadian variation in hs-CRP over the days of the week and the months of a year. The reference value for the Thai adults in the present study was 1.8 mg/L, range 0.2-7.9 mg/L. There was no significant difference in the hs-CRP concentration because of region, time, gender or age (p>0.05), nor was the value affected by time. Conclusion The determination of hs-CRP can be performed at any time and the hs-CRP value determined by this study can be used as the reference for Thai adults. (Circ J 2005; 69: 339 - 344)