Changes of diurnal variation of peak expiratory flow rate (%PEF variation) and their relationship with bronchial hyperresponsiveness (BHR) to methacholine (PC20) were evaluated in 12 children with mild-to-moderate asthma and house-dust mite allergy, during successive periods of stay in a mite-free environment at high altitude (1756 m) and at their home at sea level. The children remained at the high altitude from October until the end of December; then they spent a 3-week period at home and returned to high altitude residence in January. PEF was measured daily, in the morning and in the evening, during the 3 months' stay at high altitude and them for 10 days after the return in January. PC20 was assessed in 8/12 children, once a month from October to December, and at the return in January. Mean absolute PEF values did not change significantly throughout the study. From October to December, patients showed a significant decrease of mean %PEF variation (P = 0.04), while PC20 showed an increase (P = 0.05). After the 3 weeks at home, both %PEF variation (P = 0.03) and PC20 (P = 0.05) significantly worsened. The correlation between PC20 values and mean %PEF variation in the 2 days before and after each methacholine test was r = -0.63 (P = 0.001). Our data suggest that there is a beneficial effect of a prolonged stay in a mite-free environment, on both PEF variability and BHR, also in asthmatic children with good pulmonary function. PEF variability and bronchial responsiveness to methacholine were significantly correlated also for small changes of the two variables.