It is commonly believed that emergency department (ED) use is affected by extreme weather. To test this hypothesis, data concerning use of a pediatric ED during three seasonally diverse months was analyzed in the light of Weather Bureau information concerning daily conditions during the study months. Seven measures of extreme weather were defined: 1) extreme cold (daily high temperature less than or equal to 25 degrees F); 2) extreme heat (daily high temperature greater than or equal to 88 degrees F); 3) unusual cold (winter) with departure from normal of mean temperature less than -10 degrees F; 4) unusual heat (summer) with departure from normal of mean temperature greater than 10 degrees F; 5) precipitation greater than or equal to 0.25 inches (in water-equivalent inches); 6) stormy (thunderstorm, hail, ice, or blowing snow); 7) snow-covered (greater than or equal to 6 inches of snow on the ground). Seasonal use patterns were examined and the proportion of days with each weather factor was compared with the proportion of visits on days with the factor. The data indicate 1) season has a major affect on ED use because it affects prevalence of disease and injury; 2) extremely cold and stormy conditions significant reductions in ED use of approximately 5-20%; 3) 80-95% of expected visits are made on days with very bad weather. The data indicate that weather is a minor factor in determining ED use.