This study was undertaken to observe the effect of the preirradiation of UVA on the eryt,hematous reaction by UVB. Twenty-five healthy Korean adult male volunteers were taken as subjects. The results were as follows: 1)Preirradiation of 10J/cm and 20j/cm of UVA had no influence on the erythematous reaction induced by UVB. Preirradiation of 30 J/cm of UVA decreased the minimal erythemal dose(MED) of UVB significantly (p<0,05), and 4pJ/cm of UVA dicl very significantly(p<0. 001). 2) Preirradiation dose of UVA(x) and MED of UVB(y) had statistically significant relationship (y=33. 23 - 33 x, r= -0.39, p<0. 05). So preirradiation of UVA decreased MED of UVR in relation with the preirradiation dose. 3) For the objective criteria of skin erythematous reaction induced by the UV irradiation, MED was considered more accurate and efficient than minimal perceptible dose (MPI).