Erythema induced by ultraviolet B radiation (UVB) or PUVA (psoralen+UVA) was measured by diffuse reflectance spectroscopy (DRS) and laser Doppler flowmetry (LD) on patients receiving UV phototherapy and on healthy volunteers of skin type V and VI. The height of the 577 nm absorbance peak of oxyhemoglobin was taken as a measure of erythema intensity. DRS spectra from skin sites exposed to a series of doses were similar in shape. The dose necessary to produce a 0.05 difference in absorbance between 577 nm and 630 nm was determined to be a good indicator of minimal erythema dose (MED) in UVB. DRS measurements proved very reliable in the determination of MED or minimum phototoxic dose) of deeply pigmented subjects. In individuals with skin types V or VI, DRS detects and provides a measure for UVB and PUVA erythema and distinguishes it from melanin pigment, even when the erythema is not obvious. LD measurements lose sensitivity with increasing pigmentation and fail altogether in highly pigmented skin (types V and VI). Diffuse reflectance spectroscopic techniques also make possible the detection of sub-visual threshold reactions. DRS improved the accuracy of MED and MPD determination and the objective standardization of phototesting procedures irrespective of the subject's pigment level.