In cystinuria, renal clearance of cystine frequently exceeds creatinine clearance, suggesting net cystine secretion; and absorption of the (di) basic amino acid is impaired at the luminal membrane of the jejunal and probably also renal tubular epithelium. We studied cystine transport in vitro in jejunal biopsy specimens of eight subjects with homozygous cystinuria and in 12 controls. Cellular/medium cystine distribution ratio was reduced in cystinuria (1.36 ± 0.36 versus 5.36 ± 0.61, p<0.001). Cystine influx across the luminal membrane was normal (221 ± 48 versus 261 ± 79 pmol.h-1cm-2). Measurement of transepithelial cystine fluxes showed net absorption in controls but secretion in cystinuria. Apparent permeability coefficients were close to normal in cystinuria except that the efflux permeability at the luminal membrane was significantly increased (0.839 ± 0.22 versus 0.186 ± 0.12 h-1cm-2), and, accordingly, at the luminal membrane, the influx over efflux permeability ratio was small (1.01 ± 0.50 versus 4.95 ± 0.80, p<0.001). The defect in cystine transport in cystinuria is apparently not caused by decreased influx but increased efflux of cystine (or cysteine) from the cell to the lumen across the luminal membrane. (PediatrRes 21: 477-481,1987)