Some mycosis fungoides patients show abnormal light reactions in apparently normal skin. Clinically it is observed that lesional skin most easily burned during photochemotherapy. For this reason phototests to broad wavelength regions were performed in 9 mycosis fungoides patients, close to skin lesions and at 3 and 10 cm from cutaneous lesions. The light doses giving the minimal erythemal dose, were of about the same magnitude at a distance of 3 and 10 cm from the lesions. One third of the patients showed abnormal reactions following UVA and UVB. More abnormal reactions were seen close to lesional skin, and a displacement towards increased light sensitivity was demonstrated.