The diffusion of fluorescein into nylon-66 fibers has been studied for the first time by laser scanning confocal microscopy (LSCM). LSCM makes it possible to noninvasively obtain high-resolution three-dimensional images of the spatial distribution of dyes (fluorescein) in fibers dyed for various length of times. Integration over the dye distribution yields the total amount of dye in the fiber, which is found to be in close agreement with that determined by UV-vis spectrophotometry after dissolving the fibers. Thus, the diffusion coefficients determined noninvasively by LSCM ((6.9 +/- 1.0) x 10(-11) cm(2)/s) and the destructive traditional means ((7.8 +/- 1.9) x 10(-11) cm(2)/s) also agree. The LSCM method has several significant advantages. Among these are its speed, nondestructive nature, and the ability not only to determine the total dye content of the fiber but also to image the dye distribution profile across the fiber diameter. This latter ability is demonstrated to be important to understanding the visual appearance of dyed fibers and fabrics. Two fibers, one ring-dyed and one uniformly dyed, each with the same over all dye content, show remarkably different shades of color. The ring-dyed fiber is lighter, an observation confirmed by the reflectivities measured for each fiber, which were in the ratio ring-dyed/uniformly dyed = 2/1. LSCM observation of dyed fibers provides us not only with a means to measure the dye diffusion coefficient in the fiber, but also the time-dependent, three-dimensional distribution of dye molecules.