As part of a consideration of the sampling method for refractory ceramic fibers, calculations were carried out at the National Institute for Occupational Safety and Health to evaluate different approaches to fiber measurement. The most common technique for estimating fibers that can reach the lungs is to use an upper diameter limit of 3 microns in the phase contrast optical microscope counting rules. Calculations were carried out to estimate the aerodynamic diameter of fibers in several lognormal size distributions likely to occur in workplaces. Using these size distributions, the use of a 3 microns fiber diameter upper limit in the counting rules was compared with results expected from a sampler designed to collect fibers according to the thoracic definition, which is based on the aerodynamic diameter of compact particles. The other limits in the optical courting procedure, i.e., counting only fibers longer than 5 microns and thicker than 0.25 micron, were included in the calculations. The calculations indicate that the 3 microns upper diameter counting rule agrees with the thoracic definition within about +/- 25% for a wide range of possible fiber size distributions. The advantages of using a sampler designed to collect the thoracic fiber size fraction include reducing analyst decision making (all fibers collected would be counted) and reducing the nonthoracic particles on the sample, making the sample easier to analyze. Until thoracic samplers are available for fibrous aerosols, incorporating the 3 microns upper diameter limit as part of the criteria for counting fibers may serve as a surrogate for thoracic sampling.