A study was undertaken to assess pulmonary health effects of hydrogen sulfide (H2S) exposure in a group of workers (n = 175) extracting and processing oil and natural gas in west-central Alberta. Exposure to H2S was assessed by questioning the workers about "exposures strong enough to cause symptoms," and exposures that resulted in loss of consciousness (a "knockdown"). Exposures strong enough to cause symptoms were reported by 34% of the workers. Fourteen workers (8%) reported having had a knockdown. Exposures severe enough to cause symptoms were not associated with lower spirometric values or excess symptoms. Knockdowns were not associated with lower spirometric values but were associated with statistically significant excesses of (1) shortness of breath while hurrying on the level or walking up a slight hill (OR = 3.55; 95%CI = 1.02-12.4); (2) wheeze with chest tightness (OR = 5.15; 95%CI = 1.29-20.6); and (3) attacks of wheeze (OR = 5.08; 95%CI = 1.28-20.2). The pattern of excess respiratory symptoms is consistent with bronchial hyperresponsiveness, which has been documented in studies of high-level exposure to other irritant gases. Additional study is warranted and should include assessment of bronchial reactivity.