purpose: We identify a new syndrome of acquired painful diffuse osteosclerosis associated with past intravenous drug abuse in two adults. methods: A 28-year-old white woman and a 38-year-old black man with a history of non-A, non-B chronic active hepatitis were referred to us for increasing bone pain that was especially severe in their lower extremities. They were studied at our clinical research center. results: Skeletal radiographs documented progressive generalized osteosclerosis. Increased bone mass was confirmed by dual-energy radiography, and bone scintigraphy showed diffusely increased radionuclide accumulation. Serum biochemical studies revealed elevated alkaline phosphatase activity and osteocalcin levels, mild to moderately increased 1,25-dihydroxyvitamin D concentrations, and normal parathyroid hormone levels. In urine, hydroxyproline excretion was elevated, whereas calcium levels were reduced. Iliac crest histomorphometry showed increased rates of bone formation. Hematology, renal function, serum protein electrophoresis, and screening for fluorosis as well as vitamin A and heavy metal poisoning were all normal. Family histories were negative. Both patients were seropositive for antibody against hepatitis C virus as well as against Epstein-Barr virus (antiviral capsid antigen IgG but not IgM). Each subject was seronegative for cytomegalovirus, human immunodeficiency virus (HIV) 1 and 2, and human T-cell lymphotropic virus (HTLV) 1 and 2. Assay for reverse transcriptase in lymphocyte co-culture fluid and polymerase chain reaction studies using HIV-1 primers on peripheral monocyte DNA were negative. Treatment with synthetic salmon calcitonin in both individuals rapidly led to decreased bone pain and to a decline in biochemical parameters of accelerated bone turnover. conclusion: Painful diffuse osteosclerosis can follow intravenous drug abuse and is possibly caused by parenteral transmission of a virus that in some way stimulates bone formation.