['Pain during photodynamic therapy (PDT) is a considerable problem that needs to be studied to improve this otherwise attractive treatment of skin diseases.', 'To compare pain during PDT using two different fluence rates, and also to evaluate the association between pain and protoporphyrin IX (PpIX) fluorescence, lesion type, lesion preparation and lesion localization.', 'Twenty-six patients with actinic keratoses (AKs) in different localizations and 34 patients with facial acne vulgaris were treated with methyl aminolaevulinate-PDT. Patients with acne were illuminated using two different fluence rates. Pain score during PDT and PpIX fluorescence prior to illumination were measured.', 'The study showed that pain during illumination was associated with the PpIX fluorescence in the treatment area (P = 0.0003, R(2) = 0.31). When using a fluence rate of 34 mW cm(-2) patients with acne had a pain score of 6 [interquartile range (IQR) 5-7] compared with 8 (IQR 6-10) when using a fluence rate of 68 mW cm(-2) (P = 0.018). After correcting the pain score for PpIX fluorescence no differences in pain scores were found between first and second acne treatment, locations of AK lesions or between the two types of lesions.'] Is pain during photodynamic therapy associated with protoporphyrin IX fluorescence and fluence rate?