The relationship between quantitative data on peripheral blood cultures and source of infection was studied in 172 episodes of candidemia that occurred in 169 patients. Clinically, the source of candidemia was an intravascular device in 67 episodes, an extravascular source in 73 episodes, and endocarditis in 2 patients; no source was identified for the other 30 episodes. Colony counts were determined in peripheral blood specimens on the first day of candidemia by the lysis-centrifugation system. High-grade and low-grade candidemia were defined as 25 colony-forming units or more per 10 ml and 10 colony-forming units or fewer per 10 ml of blood, respectively. Of 48 episodes of high-grade candidemia, 43 (90%) were associated with an infected intravascular device; therefore, the presence of high-grade candidemia should prompt the removal of intravascular devices. In contrast, 92 of the 112 episodes of low-grade candidemia (82%) had an extravascular or an unidentified source of candidemia. In patients with infections associated with an intravascular device, colony counts declined significantly within 72 hours after removal of the device in the absence of antifungal therapy; failure to decline suggests an alternative source of persistent infection. Quantitative data from peripheral blood cultures may help distinguish intravascular from extravascular sources of candidemia and aid in assessing the response to the removal of infected intravascular devices.