The City and County of Sacramento undertook a study to characterize the variability of total recoverable concentrations of chromium, copper, lead, and zinc in urban runoff. These trace metals had previously been identified constituents of concern in Sacramento urban runoff discharges. The objectives of the study were: (1) to determine the variability of trace metals concentrations during the first storm of the rain season and a mid-season storm, at four different representative watersheds and (2) to evaluate the significance of the data with respect to selecting Best Management Practices (BMPs) and control strategies for reducing urban runoff pollutants. This information was needed to identify the types of runoff on which to focus measures for reducing trace metal contributions to area receiving waters. Discharges were sampled every half hour for the first 12 hours of storm runoff. The data showed a marked synchrony in concentration fluctuations among the four metals over the 12-hour time course. The pattern of metals concentration at each site was generally similar for both storms. The size of the watersheds also appeared to be a factor in the pattern of trace metals concentrations over time. The study concluded that, for both storms, the pollutant concentrations were generally highest during the first few hours of runoff, although this “event first-flush” effect was more pronounced during the first storm of the season. The concentrations of all four metals were higher in the first storm than in the mid-season storm, providing evidence of a “seasonal first-flush” effect. Thus, the amount of rainfall preceding a storm appeared to play a critical role in the build up of pollutants in the urban watersheds. The implications of this study are that there may be opportunities to control significant amounts of urban runoff pollution by directing BMPs toward first-flush effects.