Background: Acute guttate psoriasis (AGP) is a distinctive clinical entity with good response to treatment with narrow-band ultraviolet B (NB-UVB). Objective: To investigate the results of NB-UVB phototherapy in adult patients with adult guttate psoriasis. Material and Methods: We carried out a prospective, open, and observational study. Patients over 18 years with more than 5% of body surface area affected were included. The PASI was assessed prior to and after treatment. The follow-up period was 18 months. After treatment, patients completed a simple questionnaire to assess their overall impression of the treatment. Results: The 67 adult patients with AGP included in this study had an initial PASI of 8.55 (SD 5.03). Patients were treated with a mean of 19.9 sessions (SD 13.5) and mean doses of 14 mJ/cm2 (SD 10.5). Of the 67 patients, 52 achieved PASI90 with 96.15% of PASI reduction, and of these, 46 (88%) maintained PASI90 during the 18 months of follow-up. Patients were very satisfied with the treatment. Discussion: AGP is a defined clinical entity with a variable course. Phototherapy with NB-UVB appears to be a very good option for treatment of AGP because of the good results obtained and patient satisfaction.