Background: Photodynamic therapy (PDT) is an attractive therapy for nonmelanoma skin cancers and actinic keratoses (AKs). Daylight-mediated PDT is a simple and tolerable treatment procedure for PDT. Methyl aminolaevulinate (MAL)-PDT is approved for the treatment of thin or nonhyperkeratotic AKs on the face and scalp. However, thick AK lesions are often treated as well when present in the field-cancerized treatment area. Objectives: In a randomized multicentre study to evaluate efficacy of daylight-mediated PDT for different severity grades of AKs. Methods One hundred and forty-five patients with a total of 2768 AKs (severity grades I-III) of the face and scalp were randomized to either 11/2 or 21/2 h exposure groups. After application of a sunscreen (sun protection factor 20) and gentle lesion preparation, MAL was applied to the entire treatment area. Patients left the clinic immediately after application and exposed themselves to daylight according to randomization. Daylight exposure was monitored with a wrist-borne dosimeter. Results: No difference in lesion response was found between the 11/2 and 21/2 h exposure group. The mean lesion response rate was significantly higher in grade I lesions (75·9%) than in grade II (61·2%) and grade III (49·1%) lesions (P < 0·0001). Most grade II (86%) and III AKs (94%) were in complete response or reduced to a lower lesion grade at follow-up. Large variations in response rate of grade II and III AKs were found between centres. No association was found between response rate and light dose in patients who received an effective light dose of > 3·5 J cm -2. Conclusions: Daylight-mediated PDT of moderate to thick AKs was less effective than daylight-mediated PDT of thin AKs especially in some centres. However, nearly all thicker lesions (grades II and III) were reduced to a lower lesion grade at 3 months after a single treatment of daylight-mediated PDT.