Background: Hydroa vacciniforme (HV) is a rare, sporadic, idiopathic photodermatosis characterized by vesicles and crust formation after sunlight exposure. The lesions typically heal with vacciniform scarring. Objective: We identify and review the clinical features and follow-up data of Scottish patients with HV and report on the prevalence of this condition. This is the largest recent study of HV patients from a single center. Methods: In this retrospective study, patients with HV were identified by means of the diagnostic database from the Photobiology Unit, Dundee. Patients were contacted and details of clinical features, duration of disease, results of investigations, and treatment were recorded. At review, disease progress was assessed. Results: Between 1973 and 1997, 17 patients (9 males and 8 females) with a diagnosis of HV were investigated. Data from 15 patients showed a mean age at onset of 7.9 years (range, 1 to 16 years), with females (mean, 6.7 years; range, 2 to 12 years) having an earlier onset than males (mean, 8.7 years; range, 1 to 16 years). A bimodal age distribution was also identified with onsets between the ages of 1 and 7 years and 12 and 16 years. At review, spontaneous clearing had occurred in 9 patients (60%) with mean duration of disease being 9 years (range, 4 to 17 years). Males had longer disease duration (mean, 11 years; range, 5 to 17 years) than females (mean, 5 years; range, 4 to 7 years). Eight patients (53%) were sensitive in the UVA waveband on monochromator phototesting, and 6 (40%) experienced papulovesicular lesions on repetitive broad-spectrum UVA irradiation. All patients received broad-spectrum sunscreens with variable results. Of the 5 patients treated with narrow-band UVB (TL-01) phototherapy, 3 reported beneficial results with an increase in tolerance to sunlight exposure and associated reduction in disease severity. Conclusion: The estimated prevalence of HV was at least 0.34 cases per 100,000 with an approximately equal sex ratio. Males had a later onset and longer duration of disease than females. Phototesting showed abnormal responses in the UVA wavebands in 53% of cases, whereas 60% of patients treated with prophylactic TL-01 phototherapy found it beneficial. (J Am Acad Dermatol 2000;42:208-13.)