The sources of metals for the sediment-hosted massive sulfide deposits currently forming at the sediment-filled Middle Valley segment of the northern Juan de Fuca Ridge have been investigated using Pb isotopes. Leg 139 of the Ocean Drilling Program drilled three sites in Middle Valley at which basaltic rocks, sediments, and sulfide mineralization were recovered. At site 856, massive sulfides composed of pyrite, pyrrhotite, minor chalcopyrite, and sphalerite have isotopic compositions intermediate between Juan de Fuca basalts and Middle Valley turbiditic sediments, with the exception of two samples that are relatively nonradiogenic. Primary pyrrhotite-dominated and secondary pyrite-dominated sulfides overlap in isotopic composition, indicating that both high- and low-temperature hydrothermal fluids have interacted with the sediment pile. Drilling at sites 857 and 858 intersected hydrothermally altered basaltic sill-sediment complexes containing disseminated aggregates and veins of secondary hydrothermal sulfides. The sulfide minerals have highly variable, continental crust-like Pb ratios with Stacey-Kramers model ages ranging from zero to 1.5 Ga (206Pb/204Pb ~ 16.2). Some massive sulfide samples from site 856 also include an old sedimentary Pb component. The source of this old Pb is most likely Proterozoic detrital sulfides from turbidites within Middle Valley, even though the dominant source of detritus is thought to be the Mesozoic accreted terranes of Vancouver Island. The relative abundances of Cu and Zn in Middle Valley massive sulfides do not correlate with Pb isotopic composition, probably due to similar Cu/Zn in basalt and sediment components.