BACKGROUND/AIMS: The autofluorescence spectrum of skin consists of a number of overlapping single spectra emitted by fluorescent molecules. The overlapping makes it difficult to distinguish changes in the overall spectrum and the molecular basis for it. We applied curve fitting analysis to decompose the autofluorescence spectrum of normal human skin and studied the variation in different body sites. METHODS: Skin autofluorescence spectra were obtained in vivo from the normal skin of 17 subjects, using 5 excitation wavelengths: 340, 350, 360, 370 and 380 nm. The spectra were displayed in the wavelength range of 400 nm to 800 nm. Curve fitting was performed for each spectrum using the Levenberg-Marquardt algorithm. RESULTS: The skin autofluorescence spectra, provoked by all the excitation wavelengths, consisted of three component bands with center wavelengths at 450 nm, 520 nm and 625 nm. The 450 nm band contributed about 75% to the intensity of the overall spectrum, the 520 nm band contributed 25%, and the 625 nm band contributed about 2%. The three bands may correspond mainly to the fluorescence of nicotinamide adenine dinucleotide (NADH), flavin adenine dinucleotide (FAD), and porphyrins. Significant differences were found between the spectra of forehead and buttock skin. In forehead skin, the 625 nm band contributed more to the intensity of the overall spectrum, while the 450 nm and 520 nm bands had much lower peak heights, and the 450 nm and 625 nm bands were broader. CONCLUSIONS: The autofluorescence of skin is an overlapped spectra of molecules such as NADH, FAD, and porphyrins. These molecules contribute differently to the overall spectrum in different body sites.