OBJECTIVE To assess 5-aminolevulinic acid (ALA)-induced protoporphyrin IX accumulation and fluorescence in peritoneal colon carcinoma metastases and its benefits for laparoscopic fluorescence diagnosis. BACKGROUND Occult, macroscopically nonvisible peritoneal micrometastases can be missed in laparoscopy or open surgery. Laparoscopic fluorescence diagnosis allows detection of these lesions after intraperitoneal lavage with ALA and subsequent fluorescence induction by blue-light excitation. METHODS A disseminated peritoneal carcinosis was induced by laparoscopic implantation of colon carcinoma cells (CC531) in the peritoneum of 55 WAG/Rij rats. After 12 days of tumor growth the animals were randomized into 11 groups with different photosensitization parameters. Peritoneal lavage was performed either with 1.5% or 3.0% ALA solution, except for one control group. Photosensitization times were 0.5, 1, 2, 4, or 8 hours. Spectrometry was performed using an optical multichannel analyser. ALA and protoporphyrin IX serum levels were measured by high-performance liquid chromatography to determine systemic load. RESULTS Protoporphyrin IX tumor accumulation and fluorescence peaked 2 to 4 hours after ALA application in both main groups, 1.5% and 3.0% ALA. Tumor detection rate was most effective in the 1.5% ALA group. Compared with conventional white-light laparoscopy alone, blue-light excitation detected 35% additional intraabdominal tumor foci. CONCLUSIONS Laparoscopic fluorescence diagnosis can increase the sensitivity and specificity of diagnostic staging laparoscopy. It allows determination of the extent of peritoneal carcinosis. Improved preoperative assessment helps to avoid unnecessary laparotomies and radical resections.