The efficacy of narrowband ultraviolet B (UVB) was assessed in 100 consecutive patients with psoriasis by quantifying disease severity using objective (Psoriasis Area and Severity Index, PASI and Dermatologists Global Assessment, DGA) and subjective (Psoriasis Disability Index, PDI) measures. The median pretreatment PASI, DGA and PDI were 5.7 (interquartile range, IQR 4.5-8.35), 7 (IQR 6-9) and 42 (IQR 29-63.5), respectively. At 3 month follow-up, the PASI, DGA and PDI had fallen to 2.7 (IQR 1.1-3.5), 3 (IQR 2-5) and 30 (IQR 21-50.5), respectively (P < 0.001). A small group of patients continued to score highly on their PDI despite being clinically clear or having minimal disease, possibly representing chronic disability behaviour. Patients exhibiting this may require more intensive supervision. In most patients, symptoms of itch and pain improved or disappeared (70% and 75%, respectively). Side-effects were reported in 18%. Narrowband UVB phototherapy is safe and effective for psoriasis. Symptoms and subjective quality of life measures improved significantly. Both objective and subjective measures should be used when evaluating the efficacy of a treatment for psoriasis.