In Michigan, sections of an Interstate-type pavement are suffering extensive cracking and joint deterioration after 10 years of service, having been constructed in 1992. An adjacent section constructed in 1993 with comparable design features and materials remains in good condition, with little visual sign of distress. A study was conducted to determine, if possible, the cause of the observed distress in the highway built in 1992. In all, cores from nine different projects were evaluated, all of which were made with iron blast-furnace-slag coarse aggregate and natural fine aggregate containing chert constituents. The analyses conducted included stereo and petrographic microscopy and chemical extractions to determine levels of exchangeable and soluble potassium and sodium, as well as sulfates. The findings indicate that, in distressed pavement sections, the chert constituents in the fine aggregate are deleteriously alkali-silica reactive (ASR), whereas these same constituents are not deleterious in the sections rated as fair. Further, the distressed sections all had sulfate levels significantly higher than predicted by the mixture design. It is hypothesized that, in addition to the ASR in the fine aggregate, dissolution of the calcium sulfide dendrites in the slag coarse aggregate is providing excess internal sulfates, resulting in in-filling of the air-void system with ettringite and potentially sulfate attack. The exact nature of the deterioration mechanisms is not fully understood, but it seems clear that some type of interaction exists between the ASR and excess sulfates.