Coke oven and by-product workers are potentially exposed to coke oven emissions (COE), which contain hundreds of chemicals and are primarily composed of polycyclic aromatic hydrocarbons (PAH) and volatile organic compounds. Some of these compounds are hepatotoxins. The objective of this study was to examine the relationship between work in coke oven and by-product plants and serum activities of aspartate aminotransferase (AST) and alanine aminotransferase (ALT), the most commonly performed liver-function tests. The exposed group was composed of current workers who had been employed at least 3 months in the two coke-operation work areas, including one coke oven plant and one by-product plant (Area I: n = 117; Area II: n = 96) of a large steel company in Taiwan. Control subjects (Area III: n = 131), not visiting either coke-operation area in the last 3 months, were collected from the administrative and nonproduction areas in the same company. PAH exposure, as a surrogate of COE, was measured monthly by PM-10 size-selective high-volume-area air samplers in or around these three areas between June and December 1990, as well as between November 1992 and June 1993. The mean total respiratory particulate PAH exposure levels (<10 µm) between November 1992 and June 1993 in Areas I, II, and III were 6.8 × 103, 2.1 × 103, and 6.5 × 101ng/m3, respectively. AST, ALT, and hepatitis B surface antigen tests were performed in 1994. Workers who showed either AST or ALT levels greater than reference levels (abnormal >25 IU/L) were regarded as showing "elevated liver enzyme levels." Workers in Area I had AST levels that were 17% higher (95% confidence interval [CI], 3% to 32%]) and ALT levels that were 35% higher (95% CI, 10% to 65%)] than those in Area III after controlling for appropriate confounders. The adjusted odds ratio (Area I vs Area III) for elevated liver enzymes was 4.4 (95% CI, 1.5 to 13.4). In addition, coke oven (n = 91) and by-product workers (n = 26) from Area I had ALT levels 37% and 45% higher, respectively, compared with control subjects from Area III, after adjusting for appropriate confounders. Similar effects are also seen for AST. Workers in Area II had slightly, but not significantly, elevated AST and ALT levels. These results indicate that workers most heavily exposed to COE exhibit elevated aminotransferase levels.