BACKGROUND: Many studies have reported hypocalcemia during peripheral blood stem cell (PBSC) collection. On the other hand, changes in other electrolyte levels such as potassium have received little attention. To see if it is necessary to monitor other electrolytes, we determined sodium, potassium, chloride, total CO(2), and ionized calcium before and after the PBSC harvest. The changes in hemoglobin and platelets were also investigated. METHODS: A total of 111 PBSC harvest procedures for thirty-six patients or donors were included in this study. The samples were collected within 2 hours around PBSC harvest. Patients who received blood transfusions or in whom electrolytes were administered immediately before or during the procedure were excluded from the evaluation. RESULTS: In the autologous PBSC harvest using CS3000, all five electrolytes showed significant changes. Ionized calcium significantly dropped by about 7.2+/-13.1% at the end of apheresis. Potassium significantly decreased to 12.3+/-10.2% during PBSC harvest. Hemoglobin and platelets significantly decreased except the autologous PBSC harvest using CS3000 and Cobe, respectively. CONCLUSIONS: Our data reveal a strong association between anticoagulant-induced hypocalcemia and concomitant hypokalemia during the PBSC harvest. We suggest that the potassium level should be carefully monitored, especially for patients with relatively low preharvest potassium levels.