Confocal endomicroscopy is a novel and non-invasive microscopic technique that enables surface and subsurface imaging of living tissues or cells in vivo. The purpose of this study was to assess the possibility of utilizing a rigid confocal endomicroscope (RCE) system developed for detecting morphological changes in living normal and neoplastic human tongue tissue in combination with 5-aminolevulinic acid (ALA)-induced endogenous protoporphyrin IX (PPIX) fluorescence. Three patients with squamous cell carcinoma (SCC) of the tongue were examined using the novel RCE system with the excitation wavelength at 488 nm from an argon-ion laser and the detection wavelengths of the tissue fluorescence above 515 nm. Patients were topically applied with 0.4% of 5-ALA rinsing solution to the oral mucosa for approximately 15 min, and then the confocal endomicroscopic imaging of tissue PPIX fluorescence was performed on the lesion sites of the tongue after an optimal incubation period of 90-120 min. For comparison purposes, ALA-PPIX fluorescence confocal endomicroscopic imaging was also carried out on the normal sites of the tongue in vivo from two healthy volunteers. Image distortions due to tissue motion can be minimized using a specially designed tissue stabilizer attached to the RCE probe. Good quality ALA-mediated confocal fluorescence images of the tongue can be acquired in real-time, providing well-defined micro-morphological structures (e.g., filiform papillae, keratinized epithelium and fungiform papillae) of the tongue in vivo. Changes of tissue structures in oral tissue associated with cancer transformation can also be clearly identified using the RCE imaging. Preliminary results obtained in this study suggest that ALA-mediated rigid confocal endomicroscopy may have a significant potential for the rapid, non-invasive diagnosis and evaluation of early oral cancers in vivo.