Refractory ceramic fibers (RCF) are man-made vitreous fibers (MMVF) used in high-temperature industrial applications. Between 1987 and 1994, a prospective study evaluated pulmonary function of 361 male workers currently employed in RCF manufacturing and related operations for plausibility of a causal relationship between RCF exposure and pulmonary function changes. Workers included in the analysis provided at least five pulmonary function tests. The exposure–response relationship was modeled with two exposure variables: years in a production job, and cumulative fiber exposure (fiber-mo / cc). Comparison groups were nonproduction workers and workers with up to 15 fiber-mo / cc cumulative exposure. A statistically significant decrease in FVC was demonstrated among workers employed in production jobs more than 7 yr prior to initial test. A similar but nonstatistically significant result was demonstrated for FVC in workers with greater than 60 fiber-mo / cc cumulative exposure prior to initial pulmonary function test. Similar but nonstatistically significant results were obtained for FEV1. These findings, which primarily reflect workers employed before 1980, did not persist with analysis of follow-up production years and accumulated RCF exposure from initial pulmonary function test. Since longitudinal analyses are sensitive to influences that continue to affect annual decline during the study period, lower RCF exposure levels since the 1980s may be responsible for eliminating any further effect on pulmonary function.