Narrowband ultraviolet B phototherapy (NB-UVB) is a widely used modality in the treatment of psoriasis and is generally accepted as safe in pregnancy. Previous studies have described photodegradation of serum folate after exposure to UVA radiation but the effect of UVB is not known. We studied the effect of NB-UVB phototherapy on serum folic acid levels in patients with psoriasis and the relationship between changes in serum folate levels and the total cumulative dose of NB-UVB. Included in the study were 30 psoriatic patients between 13 and 55 years of age. Serum folate levels were measured at baseline, and after exposure to 18 and 36 sessions of NB-UVB irradiation. There were significant decreases in mean serum folate levels after NB-UVB exposure. After exposure to 18 and 36 sessions the decreases were 19% and 27%, respectively. After 18 sessions, the mean serum folate level had decreased in 18 patients (60%) from 8.64 ng/ml at baseline to 7.02 ng/ml (mean NB-UVB cumulative dose 40.02 J/cm(2); P = 0.019). After 36 sessions, the mean serum folate levels had decreased in 22 patients (73%) to 6.32 ng/ml (mean NB-UVB cumulative dose 118.16 J/cm(2); P = 0.002). The present study showed that high cumulative NB-UVB doses can induce folate photodegradation and decrease serum folate levels in patients with psoriasis and that this effect is directly related to the total cumulative dose of NB-UVB.