Objective: To assess the annual lung function decline associated with the reduction of secondhand smoke exposure in a high-altitude industrial workforce. Methods: We performed pulmonary function tests annually among 109 high-altitude gold-mine workers over 5 years of follow-up. The first 3 years included greater likelihood of exposure to secondhand smoke exposure before the initiation of extensive smoking restrictions that came into force in the last 2 years of observation. Results: In repeated measures modeling, taking into account the time elapsed in relation to the smoking ban, there was a 115 ± 9 (standard error) mL per annum decline in lung function before the ban, but a 178 ± 20 (standard error) mL per annum increase afterward (P < 0.001, both slopes). Conclusion: Institution of a workplace smoking ban at high altitude may be beneficial in terms of lung function decline.