In recent years peripheral blood stem cell (PBSC) collection for allogeneic or autologous transplantation has experienced an increased use in the onco-hematological setting. The latest generation cell separators allow a satisfactory and safe PBSC collection. Nevertheless, as in all therapeutic apheresis procedures, patients may experience procedure-related side-effects, mainly vasovagal reactions or symptoms related to hypocalcemia and/or hypomagnesemia. We investigated electrolyte changes in 18 patients, with a median age of 46 years (range 7-62), undergoing PBSC collection from January to April 1998. A significant decrease in total calcium in the final sample (9.65 Β± 0.7 mg/dL) with respect to the basal one (9.2 Β± 0.6 mg/dL, P < 0.05) was observed; also ionized calcium decreased markedly from the first sample drawn at +30 minutes: 1.22 Β± 0.14 vs. 1.03 Β± 0.15 mmol/L (P < 0.05), and a highly significant difference emerged when basal value were compared to the final value: 1.22 Β± 0.14 vs. 0.94 Β± 0.13 mmol/L (P < 0.0001). Similar findings affected potassium concentration: 4.1 Β± 0.4 vs. 3.3 Β± 0.3 mEq/L (P < 0.0001). Three out of eighteen patients (16.7%) reached a final potassium level <3.0 mEq/L, and eight out of eighteen (44.5%) showed a potassium concentration decrease >20% with respect to the basal value. A mild metabolic alkalosis occurred during the procedure: pH increased from 7.35 Β± 0.02 to 7.43 Β± 0.028 (P < 0.001), and plasma bicarbonate concentration increased from 27.48 Β± 2.21 to 32.44 Β± 2.52 mmol/L (P < 0.01). Sodium and chloride did not differ in the final sample with respect to the basal sample. None of our patients experienced clinically relevant side effects related to severe electrolyte changes (i.e., >20% with respect to the basal value). Because our current therapeutic schedules include patients older than 50 years in the PBSC collection and transplantation program and since it is well known that subclinical myocardial disease may occur in up to 4% of middle-aged males, we suggest that patients aged 50 or older undergoing PBSC collection procedures be carefully monitored in order to identify significant electrolyte variation, especially if they present with low serum potassium levels. However, further investigation of larger patient series are needed to determine the clinical relevance of serum potassium changes during apheresis.