Background Photodynamic therapy (PDT) with 5-aminolaevulinic acid (ALA) or its methylester [methyl-5-aminolaevulinate (MAL) or 5-amino-4-oxopentanoate] was recently ranked as first-line therapy for the treatment of actinic keratosis (AK) and is an accepted therapeutic option for the treatment of neoplastic skin diseases. BF-200 ALA (Biofrontera Bioscience GmbH, Leverkusen, Germany) is a gel formulation of ALA with nanoemulsion for the treatment of AK which overcomes previous problems of ALA instability and improves skin penetration. Objectives To evaluate the efficacy and safety of PDT of AKs with BF-200 ALA in comparison with a registered MAL cream and with placebo. Methods The study was performed as a randomized, multicentre, observer-blind, placebo-controlled, interindividual trial with BF-200 ALA, a registered MAL cream and placebo in a ratio of 3 : 3 : 1. Six hundred patients, each with four to eight mild to moderate AK lesions on the face and/or the bald scalp, were enrolled in 26 study centres in Germany, Austria and Switzerland. Patients received one PDT. If residual lesions remained at 3 months after treatment, PDT was repeated. Results PDT with BF-200 ALA was superior to placebo PDT with respect to patient complete clearance rate (78·2% vs. 17·1%; P < 0·0001) and lesion complete clearance rate (90·4% vs. 37·1%) at 3 months after the last PDT. Moreover, superiority was demonstrated over the MAL cream regarding the primary endpoint patient complete clearance (78·2% vs. 64·2%; P < 0·05). Significant differences in the patient and lesion complete clearance rates and severity of treatment-related adverse events were observed for the narrow- and broad-spectrum light sources. Conclusions BF-200 ALA is a very effective, well-tolerated new formulation for AK treatment with PDT and is superior to a registered MAL medication. Efficacies and adverse events vary greatly with the different light sources used.