Recent data suggest that responsiveness to methacholine continues to improve 2 and more years after cessation of exposure to agents causing occupational asthma (OA). The goal of this study was to characterize further the curve of improvement to methacholine responsiveness in subjects with OA. Eighty subjects with confirmed OA who had at least two assessments of a provocative concentration of histamine causing a 20% drop in FEV(1) (PC(20)) and were seen for at least 2 years after cessation of exposure. The shape of recovery of PC(20) was assessed by CARMA (James K. Lindsey, Liège, Belgium) analysis. Slopes of recovery were compared in the first 2.5 years in 55 subjects and from 2.5 years until the end of observation in 56 subjects. Recovery curves showed progressive improvements in PC(20) significantly influenced by time lapse since end of exposure, sex, baseline PC(20), and FEV(1). The slopes of recovery were significantly different from zero both for the first 2.5 years after cessation of exposure (0.27 +/- 0.05 SEM natural logarithm of PC(20) per year) and later (0.09 +/- 0.008 SEM natural logarithm of PC(20) per year), with the slope significantly steeper for the first 2.5 years. This study shows that improvement in responsiveness to methacholine continues for years after cessation of exposure but that the improvement is more rapid in the first 2.5 years.