It is now universally accepted that utilization of lead for domestic purposes and water distribution presents a major health hazard. The ancient Roman world was unaware of these risks. How far the gigantic network of lead pipes used in ancient Rome compromised public health in the city is unknown. Lead isotopes in sediments from the harbor of Imperial Rome register the presence of a strong anthropogenic component during the beginning of the Common Era and the Early Middle Ages. They demonstrate that the lead pipes of the water distribution system increased Pb contents in drinking water of the capital city by up to two orders of magni-tude over the natural background. The Pb isotope record shows that the discontinuities in the pollution of the Tiber by lead are intimately entwined with the major issues affecting Late Antique Rome and its water distribution system. harbor geoarcheology | paleopollution | Late Holocene | ore provenance | sedimentology S tatistics on demography, money supply and metal circulation, life and health standards, and many other social parameters required to understand modern history are largely missing from the written record of the ancient past. For example, the ap-parently simple question of how the population of ancient Rome evolved is still unresolved (1, 2), prompting the design of indirect estimates (3). Another well-publicized problem illus-trating the lack of primary sources of accurate information is the decade-old debate on Pb poisoning of the high society of Rome, either by lead water pipes or grape juice concoctions prepared in lead cups (4–9). Here we focus on the condition of Pb in the public waters of ancient Rome. Lead is regarded as a powerful and ubiquitous indicator of the manufacturing status of a society. For example, a surge in Pb concentrations in the Greenland ice-core record was correlated with the height of the Roman Empire (10). Three out of the four existing Pb isotopes are rapidly modified by the radioactive decay of nat-ural uranium or thorium over geological time. The mining of ores from geologically diverse areas produces metallic Pb with variable isotopic abundances that depend on the tectonic age and the Th/U and U/Pb ratios of the mining district. Arche-ologists interested in the provenance of artifacts routinely tap this wealth of information (11). To explore how the supply of metals from all over the Roman world and their utilization may have affected the nearby environment of ancient Rome, the present work sets out to investigate the isotope compositions of Pb in sediment cores from the Trajanic harbor basin at Portus, the maritime port of Imperial Rome, and the channel connecting Portus with the Tiber (Canale Romano) (Fig. S1). Harbors are excellent sedimentary traps. The record of human Pb pollution from the time that the harbor basin was excavated (ca. 112 AD) and well into the Middle Ages offers a new his-torical, ca.