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Background: Actinic keratoses (AKs) are often treated separately, lesion by lesion. However, in the past years, AKs have been described as a field disease and not limited to single clinically apparent lesions. Treatment should therefore target an area of field change which may reduce the risk of development of further AKs, second tumours, and local recurrence., Objective: The primary objective was to determine the number of new lesions at 9 months after methyl aminolevulinate photodynamic therapy (MAL-PDT). Secondary objectives were to determine the number of new lesions at 3 and 6 months after treatment and the percentage reduction of AKs from baseline at 3, 6, and 9 months after MAL-PDT., Methods: This was a single-centre, prospective, randomized, split-face, investigator-blinded pilot study with a study duration of 1 year. The study population comprised patients with AKs on the face or scalp, with a maximum of 10 AKs on each side. One side was treated with 1 session of "lesion-by-lesion" MAL-PDT (LT side) and the other side with 1 session of field MAL-PDT (FT side)., Results: At 9 months the FT demonstrated significantly fewer new lesions. At every time point during the follow-up, we found a significant reduction in the number of AKs in the LT as well as in the FT sides. After 3 and 6 months we did not observe significant differences between the sides. However, after 9 months, the LT area showed significantly fewer remaining AKs, whereas the FT area demonstrated significantly fewer new lesions., Conclusions: Field treatment results in significantly fewer new AK lesions compared with lesion-by-lesion treatment., (© 2017 S. Karger AG, Basel.)