Objective: To assess the extent to which UV-A and UV-B radiation can penetrate the human fingernail plate. Design: The Dermalite UV light machine (National Biological, Beachwood, Ohio) was used as the source of UV radiation. The amount of UV-A and UV-B penetrating the nail plate was measured using a radiometer and compared with a control. Setting: Academic phototherapy clinic. Patients: Ten cadaver fingernails were obtained from 1 cadaver from the National Disease Research Interchange. Because the objective was to determine transmission through normal fingernails, grossly diseased or deformed nails were not used. Main Outcome Measures: The percentage of UV light penetration through each fingernail was calculated by dividing the amount of radiation measured when the fingernail was in front of the light by the amount of radiation measured when there was nothing in front of the light (UV with nail divided by UV without nail). Results: All 10 fingernails completely blocked the UV-B light, reading 0 mW/cm2 on the radiometer. The mean penetration of UV-A light through the fingernails was 1.65%, ranging from 0.56% for the right fifth digit to 2.43% for the left second digit. Conclusions: The nail plate completely blocked UV-B light, and only a minimal amount of UV-A light penetrated the nails. If UV is required to directly penetrate the nail to treat nail bed psoriasis, then these data suggest that therapeutic efficacy may be compromised by the intervening nail plate. This minimal penetration of UV-A light may explain why therapies such as psoralen-UV-A (PUVA) have low efficacy for the treatment of nail psoriasis.