Chronic actinic dermatitis (CAD) is an uncommon, eczematous photosensitive eruption affecting predominantly elderly men and to which drug-induced photosensitivity may sometimes appear clinically identical. This retrospective study compares the monochromatic irradiation results in 11 patients with CAD and 14 patients with drug-induced photosensitivity, to assess whether such testing is useful in the differentiation of these two conditions. Thus, the action spectra of the drug photosensitivity patients were plotted and compared with those of 12 nonphotosensitive control patients: 10 patients were found to be photosensitive in the UVA range; the implicated drugs included quinine, sparfloxacin, amiodarone, doxycycline, mefenamic acid, nalidixic acid, fenbrufen, diclofenac, enalapril, diltiazem and prochlorperazine maleate. One patient on doxycycline was photosensitive in both the UVA and UVB ranges. The remaining three patients were not tested until after discontinuation of their drug and their light tests were then normal. In the CAD group, five patients were photosensitive in the UVA, UVB and visible light ranges and six were photosensitive in the UVA and UVB ranges. Comparison of the mean minimal erythema dose responses then demonstrated dissociation of the drug-induced from the CAD group in the UVB region; the result was statistically significant. This suggests that UVA-sensitivity dissociated from UVB-sensitivity is a relative indicator of drug-induced photosensitivity and monochromatic irradiation testing may therefore be helpful in the differentiation of these two disorders.