Twenty-two British electronics workers suffering from occupationally-induced asthma due to their exposure to colophony fumes self-recorded their peak expiratory flow (PEF) every 1-2 hr throughout the waking span for at least two weeks, both during work and vacation periods. Data were analyzed by a two-step procedure using the statistical techniques of Single and Population Mean Cosinor analyses. The objective endpoints of mesor (24-hr mean), amplitude (measure of 24-hr rhythmic variation) and acrophase (peak time along the scale of 24 hr) for the time-spans of the work-week (Monday-Friday), weekend after work (Saturday-Sunday) and vacation weekdays and vacation weekends were compared. The PEF data of four workers who participated in the study for nearly one year also were evaluated for seasonal variation. A statistically significant difference (P less than 0.04) was detected in the PEF amplitude between work and vacation weeks; the difference in the PEF mesor and acrophase between work-weekdays and work-weekends achieved near or statistical significance (P = 0.07 and 0.05, respectively). The at-work amplitude was higher and the acrophase earlier timed. There was no statistically significant difference between work-week and weekend-after-work PEF mesors. Winter PEF mesors were significantly lower than summer ones in the four subjects self-measuring their bronchial patency over the year. Overall, the findings indicate that data obtained through employee surveys, using lightweight portable and inexpensive PEF Wright meters and analyzed by statistical analyses used in the field of biological rhythm research, yield objective evidence of occupationally associated decrements in pulmonary function resulting from colophony fume exposure.