Urban stormwater runoff contains various pollutants that degrade downstream water quality. Gutter filters, below-grade filtration devices that capture sheet flow, are an ideal stormwater control measure for urban retrofits because of their small footprint. A 10-year-old gutter filter system in Mt. Rainier, MD was monitored for 18 storm events over 13 months for total suspended solids, nitrogen, phosphorus, and copper, zinc, and lead in the downstream stormwater. The filters had received no maintenance since their construction. The stormwater quality was compared to studies conducted prior to installation and immediately after installation of the filters. Total Kjeldahl Nitrogen concentrations displayed a statistically significant increase since installation. All other pollutants did not show a significant change over the 10 years. Nonetheless, overall runoff water quality was not good. Event mean concentrations are comparable to highway runoff and annual pollutant loadings are comparable to untreated runoff from other urban drainage areas in the region.