SummaryA disorder of calcium (Ca2+) metabolism may be central to the pathogenesis of cystic fibrosis (CF). Average cellular Ca2+levels in fibroblasts derived from patients with CF (ages, 14-25 yr;n=25) were 36-77% higher than in matched controls depending on age of cell culture (9.0-10.6versus5.1-7.8 nmol/mg cellular protein). Cellular Ca2+was significantly elevated in CF, but was not a reliable criterion for identifying CF cells because of the high variability of results. Studies of Ca2+fluxes in cell organelles showed that mitochondria isolated from CF fibroblasts accumulate 2-3 times more Ca2+than controls [79.5 ± 8.2versus33.7 ± 4.7 nmols · mg mitochondrial protein-1· min-1(±SD)] Ca2+accumulation in mitochondria reliably distinguished between CF and control or heterozygote cells (P<0.0005,n=11).In vitroexperiments showed that Ca2+influx and efflux are increased in isolated CF mitochondria, resulting in net Ca2+accumulation. Ca2+uptake in mitochondria is energy-dependent; some inhibitors of mitochondrial energy metabolism (atractyloside, oligomycin) influenced Ca2+uptake significantly more in CF than in control mitochondria. Furthermore, the average activities of NADH oxidase, NADH- and succinate-cytochrome c reductase were 77, 58, and 48% higher in CF mitochondria, respectively. This indicates that many functions associated with energy metabolism and the mitochondrial membrane (electron transport, ATP transport, and ATP hydrolysis) are not operating properly in CF, thus possibly causing the derangement of Ca2+metabolism found in CF mitochondria and cells.