PCDD/Fs were measured in 16 sections of a sediment core from a freshwater lake in rural England. Each section represented 10 yr of deposition. Concentrations greater than the limit of detection were observed for most homologues in all samples. Three eras of PCDD/F input were identified by their distinct homologue and isomer profiles. ∑(4-8)CDD/F levels in core sections deposited before 1900 were in the order of 100 pg/g dry weight (dw) as compared to 1500 pg/g dw in the most recently deposited sediment. Local industries such as mining, quarrying, charcoal burning, and iron smelting appear to have had a minor impact on the PCDD/F deposition in the lake. Since 1900, two major peaks in PCDD/F input to the lake were evident. The first, reaching a maximum in the 1930s, had an unusual homologue pattern dominated by high molecular weight PCDFs, and the source of this input is unknown. The second, with a maximum in the 1970s, is in keeping with previously reported time trends for Europe and North America. Pre-1900, the TCDD/F isomer pattern was dominated by dimerization products of 2,4-dichlorophenol. Concentrations of P(1-3)CDFs appeared to be connected with the input that peaked in the 1930s, while highest concentrations of P(1-3)CDDs were found in the deepest sediment sections. Concentrations of diCDDs were observed to have increased again over the most recent decades to levels similar to pre-1900 sections. Despite detailed knowledge of the catchment and of industry in the surrounding area, the identity of some sources and the contribution of other known sources remain unclear for each era. This indicates that there remain significant gaps in our understanding of PCDD/F sources and undermines our ability to predict future trends in PCDD/F emissions.