Our objective was to examine the effect of ambient temperature, age, and gender on the incidence of emergency department (ED) renal colic visits. We retrospectively analyzed a database of 15 New Jersey EDs from January 1, 1996 to December 31, 2002. We analyzed the number of renal colic visits as a fraction of total visits in monthly intervals. We used the Chi-squared test and Pearson's correlation coefficient, with P<.05 taken as statistically significant. Of the 3.5 million patient visits in the database, 30,358 (0.9%) had renal colic. Renal colic visits were 16% more likely in warmer than colder months (P<.001) and this effect was greatest in older patients and males. We conclude that higher ambient temperature, older age and male gender are associated with increased incidence of ED renal colic visits. Advice to patients, especially older males, to avoid dehydration particularly during hot weather may help prevent bouts of renal colic.