The aim was to investigate the effects of the 3-h mean concentration of suspended particulate matter (SPM) on peak expiratory flow (PEF) in asthmatic children. Subjects were 4- to 11-year-old boys and girls with asthma in Fukuoka, Japan. Daily measurements of PEF were made between April 2002 and March 2003. PEF was measured morning and evening. To assess the association between 3-h mean concentrations of SPM and morning PEF and evening PEF separately, we used generalized estimating equations. All models took into consideration seasonal effects: modified effects were examined using a two-level indicator variable for the warmer months (April through September) and the colder months (October through March). Among the 70 asthmatic children, 44 were boys. In warmer months, declines in morning PEF for 10 microg/m(3) differences in 3-h concentration of SPM measured at 2 a.m. to 5 a.m. of the same day and 11 p.m. to 2 a.m. and 8 p.m. to 11 p.m. of the previous day were -0.78 L/min (95% CI: -1.40, -0.16), -0.61 L/min (-1.18, -0.05) and -0.73 L/min (-1.32, -0.15), respectively. Results were robust even after adjustment for other air pollutants. We also found that declines in evening PEF were weakly associated with increases in 3-h concentrations of SPM in warmer months, but the associations were not statistically significant. In colder months we did not find any robust association between SPM and morning/evening PEF. In conclusion, an increased 3-h concentration of SPM is associated with declines in PEF in warmer months.