Glycocalicin (GC) is the carbohydrate-rich portion of platelet membrane glyco-protein Ibα that can be cleaved from circulating platelets by proteases. The plasma GC level is an indicator of platelet turnover. Using an ELISA for GC, we assayed the plasma of 20 normal children (age 6 to 13 years), 50 HIV+ children (ages 4 to 18 years), 32 normal adults (ages 21 to 53 years), and 50 HIV+ adults (ages 24 to 66 years). The results were adjusted for individual platelet counts to give GC indexes (GCI). The normal children and the normal adults had significantly different GCI distributions (P = .002). In both normal and HIV+ individuals the GCI decreased with increasing platelet count (-.73 < r < -.34). Twenty-eight percent of the HIV+ children and 28% of the HIV+ adults had elevated GCI values. The majority of these elevated values occurred in patients with platelet counts >100,000/µL. Neither the GCI nor the platelet count was correlated with viral load. The platelet count, however, was weakly correlated with the CD4 count in both children (r = .31) and adults (r = .30) infected with HIV. Also, the CD4 count was weakly and inversely correlated with GCI in HIV+ adults (r = -.34) and in children (r = -.24). We conclude that increased GCI and, by implication, increased platelet turnover is a relatively common feature of advanced HIV disease. Furthermore, GCI may be elevated in HIV+ patients even with a platelet count >100,000/µL, suggesting increased platelet turnover before thrombocytopenia develops.