We have compared bathing in a thermal lagoon in Iceland, combined with UVB treatment, to UVB treatment only in an open comparative study. Twenty-three psoriasis patients bathed 3 times daily and were treated with UVB 5 times a week for 4 weeks. The control group was only treated with UVB 5 times a week for 4 weeks. Psoriasis Area and Severity Index (PSAI) was used to estimate the severity of the disease. The mean PASI score in the bathing group decreased from 20.8 to 2.8 (p < 0.01). In the control UVB group, the PASI score decreased from 16. 7 to 6.9. The percentage difference between the groups was significant after 1, 2, 2 and 4 weeks. Bathing in the lagoon combined with UVB was found to be a very effective treatment and better than UVB treatment in our control group.