Predictors of sensitivity to ultraviolet (UV) light, including Fitzpatrick's sun reactive skin types, constitutive skin color and facultative skin color were assessed in 43 healthy Thai volunteers. These predictors were compared with one another and with responses of the skin to UV irradiation, as determined by a minimal erythema dose (MED) of UVB, a minimal immediate pigment darkening dose (MIPDD) of UVA, and a minimal delayed tanning dose (MDTD) of UVA, and by dose-response curves for erythema and pigmentation, as measured objectively with the use of a narrow-band spectrophotometer. The skin type did not correspond well to the constitutive and facultative skin color. There was no correlation between skin type and MED and no relation between skin type and the slope of the dose-response curves for erythema and pigmentation. Constitutional skin color was also not a good predictor of the measured MED, MIPDD and MDTD values but did appear to correlate with the steepness of the dose-response curves for erythema but not for pigmentation. The facultative skin color, however, did not correlate with the dose-response angle of erythema or pigmentation. We have thus found that the skin type is not a good predictor of UV responses in Thai. The constitutive skin color is a better predictor of UV responses of the skin than skin type and that steepness of dose-response curves for erythema is a better measure of the response of the skin to UV irradiation than is a MED measurement.