Chronologies of particle-associated contaminant levels were developed for Jamaica Bay, an embayment of the Atlantic Ocean on southwestern Long Island. A major potential source of contaminants to the system is wastewater discharge, which comprises most of the freshwater input to the bay. Sediment core sections were analyzed for Cs-137, Pu-239, 240, and Be-7 activities which were then used to establish net particle accumulation rates. Samples deposited from the early 1950s through the late 1980s were analyzed for metals and chlorinated hydrocarbon contaminants. Trace metals, including copper, lead, chromium, zinc, and mercury, were elevated to levels several times those of pre-industrial concentrations. These metals decreased by about 50% between the mid 1960s and the late 1980s. Chlorinated hydrocarbon concentrations, including polychlorinated biphenyls, chlordane, and dichlorodiphenyltrichloroethane-derived compounds, decreased by a factor of five to ten between the late 1960s and the late 1980s. Local improvements in wastewater treatment and national efforts to regulate the uses and releases of specific chemicals are the most likely explanation of declines in contaminant levels in these sediments.