Previous attempts to determine the action spectrum or spectra of the polymorphous light eruptions have been restricted by: (1) the difficulty of reproducing the clinical reactions under controlled conditions, and (2) the relatively small number of patients available for evaluation. The bulk of evidence to date indicates that the sunburn spectrum is responsible for the majority of these reactions.1-4 In a few cases longer wavelengths appear to have been responsible.2,5,6 Whether this longer wavelength group should be included in the polymorphic category is open to question. This study was designed to establish accurately the action spectrum and energy requirements necessary to reproduce the polymorphous light reactions under controlled conditions in 29 patients suffering with this disease. Materials The subjects consisted of 29 patients with polymorphous light eruptions in whom positive phototests were produced under controlled condtions (Table 1). Only reproduction of the clinical picture at the test site