Hexylaminolevulinate-mediated photodiagnosis of superficial bladder cancer recently gained marketing authorisation in Europe and is therefore being used increasingly, especially to visualise flat tumours such as carcinoma in situ. Although no significant adverse effects related to the drug have been reported to date, precise information with respect to the safety of this procedure in clinical practice has not yet been published. In the present study, we investigated the possible systemic absorption of different hexylaminolevulinate concentrations and characterised the adverse effects after bladder instillation. Twelve patients with known bladder cancer received a 2-hour administration of 50 mL of phosphate-buffered saline containing 4 mmol/L, 8 mmol/L or 16 mmol/L of hexylaminolevulinate. Safety was primarily evaluated by monitoring adverse effects. Standard haematology and clinical biochemistry were assessed by the local hospital laboratory. Measurements of hexylaminolevulinate, 5-aminolevulinate and protoporphyrin IX in plasma and solution collected after instillation were performed, as were fluorescence measurements in the urothelium and the skin by using an optical fibre-based spectrofluorometer. No hexylaminolevulinate was found in the plasma. Furthermore, 5-aminolevulinate and protoporphyrin IX showed no significant increases in plasma after intravesical instillation of hexylaminolevulinate 4 mmol/L, 8 mmol/L or 16 mmol/L for 2 hours. This implies a very low systemic absorption at the administered doses with a hexylaminolevulinate uptake from the bladder of about 5%. Neither skin sensitivity nor adverse reactions that could be attributed to the drug were reported. Renal and liver function were not affected by the hexylaminolevulinate doses used in this study. We demonstrated for the first time that hexylaminolevulinate-mediated photodiagnosis is a safe procedure for a patient undergoing this examination, the drug being only minimally systemically absorbed after intravesical instillation. Therefore, no or only minimal adverse effects compared with those seen with conventional photosensitising agents can be expected.