To determine the effect of chronic exposure to ammonia on pulmonary function among ammonia workers, 77 workers were randomly selected from an ammonia factory in the Eastern Province of Saudi Arabia and 355 were selected as controls from the administrative staffs of four industrial groups in Eastern Province. Spirometry was carried out and FEV1, FVC, and FEV1/FVC% were calculated. The ammonia level in the working environment was determined spectrophotometrically. 30% of the air samples had ammonia concentrations that exceeded the threshold limit value. Significant reductions in FEV1 % predicted and FVC % predicted were observed in ammonia workers exposed to higher cumulative ammonia levels (above 50 mg/m3-years). FEV1% predicted and FEV1/FVC% were significantly lower in symptomatic than in asymptomatic workers in the exposed group. These findings may raise the possibility that exposure to a high cumulative ammonia level produces a combined restrictive/obstructive ventilatory defect.