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Actinic erythema Polymorphous light eruption Tristimulus colorimeter Charles R. Taylor, MD, Instructor in Dermatology, Medical Director of Phototherapy, Massachusetts General Hospital, Bartlett Hall, Room 414, 32 Fruit Street, Boston, MA 02114 (USA) Garigue et al. [1J recently administered routine minimal erythema doses to 72 patients. They then used a tristimulus colorimeter to measure the erythema in 3 different modes: L*, a* and b*. Mode L* corresponds to the sample luminescence envisioned as a point along a black-white axis, whereas a* and b* are chromaticity coordinates for the red-green and yellow-blue axes, respectively. Not surprisingly, measures with mode L* were able to differentiate skin color categorized as light, intermediate or dark. Surprisingly, however, noninva-sive measures with mode a* distinguished between healthy subjects and those with a photodermatosis, namely either polymorphous light eruption or benign summer light eruption. While the minimal erythema doses appeared normal clinically, they were abnormal compared to those of nonphotosensitive volunteers on chromometric measurements. These observations may indeed reflect interesting pathophysiological mechanisms. It is important to remember that so far the authors have not uncovered a way to distinguish between a photosensitive and nonphotosensitive individual based on a single erythema reading. To me, these results merely say that if one looks at a group of photosensitive patients, their values in the a* mode tend to differ from those of nonsensitive individuals. They have yet to determine the threshold value for disease. Would the authors care to venture a number value which tends to discriminate between sensitive and nonsensitive individuals? Furthermore, it remains to be seen if these results can be duplicated in other series and whether chromometric differences in actinic erythema will be uncovered in patients suffering from additional types of photosen-sitivity. Finally, the fact that there was no difference between polymorphous and benign summer light eruptions in these measurements would seem to lend further support to the belief that the latter is merely a minor clinical variant of the former. I wondered how the authors felt about this interpretation. Reference 1 Garigue J, Marguery MC, Malmary MF, El Sayed F, Bazex J: Measurement of actinic erythema in healthy subjects and in subjects with polymorphous light eruption using a tristimulus colorimeter. Dermatology 1995; 190: 31-34.