Municipal refuse incinerator workers may be exposed to mutagenic compounds from combustion gases and particulates during plant operation, maintenance, and ash removal procedures. The frequency of mutagens was measured by the Ames assay in 3 urine samples collected from each of 37 workers in 4 refuse incinerators and 35 (control) workers from 8 water treatment plants during June-August 1990. When comparing the first urine samples contributed by workers in each cohort, incinerator workers had a significantly (p < .05) increased risk of both direct-acting mutagens and promutagens (8/37 or 22% for each mutagen type) compared with water treatment workers (2/35 or 6% for each mutagen type). Smoking within 24 h before urine sampling was not a confounder of these results. Interestingly, there was no significant (p > .05) difference for risk of urinary mutagens or promutagens between the two cohorts when comparing, respectively, the second and third urine samples from each cohort. The repeatability of demonstrating urinary mutagens in individual incinerator workers was poor, suggesting that their exposure was highly variable and/or that these workers modified their exposure (e.g., wore masks) as a consequence of being studied. Factors that influence production of mutagenic compounds during refuse incineration and subsequent worker exposure are discussed.