This study evaluated effects on respiratory health of forest firefighters exposed to high concentrations of smoke during their work shift. This is the first study of cross-shift respiratory effects in forest firefighters conducted on the job. Spirometric measurements and self-administered questionnaire data were collected before and after the 1992 firefighting season. Seventy-six (76) subjects were studied for cross-shift and 53 for cross-season analysis. On average, the cross-season data were collected 77.7 days after the last occupational smoke exposure. The cross-shift analysis identified significant mean individual declines in FVC. FEV1, and FEF25 75. The preshift to midshift decreases were 0.089 L, 0.190 L, and 0.439 I/sec. respectively, with preshift to postshift declines of 0.065 L, 0.150 L, and 0.496 L/sec. Mean individual declines for FVC, FEV1 and FEF25 75 of 0.033 L, 0.104 L, and 0.275 I/sec. respectively, also were noted in the cross-season analysis. The FEV1 changed significantly (p < 0.05). The use of wood for indoor heat also was associated with the declines in FEV1. Although annual lung function changes for a small subset (n = 10) indicated reversibility of effect, this study suggests a concern for potential adverse respiratory effects in forest firefighters.