Chronic exposure to a weak irritant leads to inflammatory changes which may be followed by pigmentary changes and accommodation. The inflammatory responses to acute exposure to an irritant have been extensively studied. This study investigated quantitatively the inflammatory reactions produced in photodamaged skin with chronic application of a weak chemical irritant (tretinoin cream 0.025%) over a period of 9 months (36 weeks). Forty-eight subjects with moderately to severely photodamaged skin were enrolled in a 36-week, double-blind placebo-controlled study. Tretinoin cream was applied nightly on the distal two thirds of one dorsal forearm and placebo on the other. The proximal third of each dorsal forearm received no treatment and served as control. Clinical assessments and diffuse reflectance measurements were made at 7 time points during treatment. Apparent concentrations of oxyhemoglobin (HbO2), deoxyhemoglo-bin (Hb) and melanin were estimated by analysis of the diffuse reflectance spectra. No changes were observed in the apparent H bO2or the Hb concentration of the placebo-treated or control sites, thus establishing a reliable baseline. The apparent HbO2 concentration of the tretinoin-treated sites increased significantly from baseline to a maximum at 12-18 weeks of treatment, then returned to baseline with continued applications. The changes in HbO2 concentration agreed closely with clinical assessments of erythema. The apparent melanin concentration, corresponding to diffuse hyperpigmentation, showed a large seasonal decrease in both the control and the treated sites, with an additional decrease in the treated sites between 12 and 18 weeks. Erythema appeared after repeated applications and eventually resolved under continuous treatment. The maximum decrease in hyperpigmentation occurred simultaneously with the maximum increase in erythema.