In an open study, the local and systemic side effects and pharmacokinetics of 5-aminolevulinic acid (5-ALA) and the fluorescent metabolite protoporphyrin IX (PPIX) were investigated after intravesical administration for the fluorescent photodetection of superficial bladder carcinoma.In 20 patients with confirmed bladder carcinoma, 5-ALA was introduced into the bladder 2 h (15 patients) and 4 h (5 patients) before an elective endoscopic resection. The 5-ALA and PPIX levels in the plasma were determined before and up to 10 h after application, and in the urine 2 h or 4 h after application.The plasma level of 5-ALA rose rapidly, the maximal concentration (340 ng/ml) being reached in 0.55 h (2 h) or 0.62 h (4 h). The elimination half-life of 5-ALA amounted to 0.74 h (2 h) or 0.79 h (4 h). In five of the patients, there was a measurable plasma concentration which ranged from the detection limit of 4.3 ng/ml to 14 ng/ml between 2 h and 5 h after application, and then fell below the detection limit after 9 h. Absorption of 5-ALA by the bladder was low, i.e. less than 1% of the total amount applied. During a period of observation of 96 h, no 5-ALA-specific side effects appeared.Because of the small quantity of 5-ALA resorbed following its intravesical administration, only minimal concentrations of PPIX that are responsible for producing side effects can be metabolised in the plasma. Therefore, no systemic side effects are to be expected after the intravesical administration of 5-ALA.