Carcinoma in situ (CIS) of the bladder is a confounding disease that is difficult to recognize endoscopically because it is a flat cancer. Many studies have suggested its relationship with subsequent invasive disease. Early recognition of CIS therefore is essential in offering the patient the most appropriate treatment and the highest cure rate. Because white light cystoscopic examination is not sufficient to reveal areas of dysplasia or CIS, random biopsies are recommended. The authors evaluate whether amino levulinic acid (ALA) fluorescence detection could be helpful in diagnosing CIS and if the specificity could be enhanced by reducing the ALA dose. Sixteen patients with papillary bladder cancer, and CIS and dysplasia were given low-dose ALA. Fluorescence detection of the metabolized ALA was performed 3 hours later, with the naked eye, after blue light illumination. Carcinoma in situ or dysplasia was found in 50 biopsies. The sensitivity for detecting CIS was 94% with a specificity of 54%. Carcinoma in situ can be diagnosed with a very high accuracy through fluorescence detection after ALA instillation. Fluorescence detection can be achieved with the naked eye and does not necessitate either complex equipment or specially trained personnel.