BACKGROUNDS - Ultraviolet radiation B (UVRB) is the most important environmental factor capable of altering the immune function of human skin. OBJECTIVE: To evaluate the association of the phenotypes of susceptibility or resistance to ultraviolet radiation B (UVRB) and the polar forms of leprosy. MATERIAL AND METHODS: We evaluated 38 patients with lepromatous leprosy (LL) and 87 patients with tuberculoid (TT) leprosy, according to the classification by Ridley and Jopling (1966). All the patients were submitted to a test to determine the phenotypes of susceptibility or resistance to UVRB through the application of a 2% dinitrochlorobenzene (DNCB) disc to a previously irradiated area with twice the minimal erythema dose (MED). After 21 days, a similar disc soaked in 0.05% DNCB was applied to the scapular area (unexposed to UVRB) to check for sensitiveness, with reading of the results after 48 hours. The patients that showed a positive reaction to DNCB were considered resistant (UVB-R) and those who did not show any reaction were considered susceptible (UVB-S). RESULTS: The frequency of UVB-S individuals was 63.2% (24 patients) in the LL group and 34.4% (30 patients) in the TT group (OR=3.26; IC=1.36 - 7.87; x(2)=7.73; p=0.005). CONCLUSION: Our results suggest that UVB-susceptibility is a risk factor to the development of lepromatous leprosy (LL).856839842