Central venous catheters are being used with increasing frequency to administer drugs, and as a result, catheter obstruction caused by precipitation of poorly soluble fluid components has become a common problem. We report our first experience using 0.1 N hydrochloric acid to restore patency to central venous catheters obstructed from insolubility-induced precipitation. Precipitation was caused by drug as well as calcium and phosphorus incompatibilities. The initial use of urokinase in two cases was unsuccessful in restoring catheter patency. In all four cases, the instillation of 0.2-1.0 ml of HCl cleared the catheters. Catheter patency usually was gained immediately. No side effects were noted. Our experience supports preliminary data (JPEN 9 (suppl):255, 1985) which suggest that 0.1 N HCl is effective in clearing insolubility-induced precipitation in central venous catheters.