This study was aimed at measuring concentration of electrolytes, especially K+ in expressed prostatic secretion (EPS) and urine from patients with chronic prostatitis. The concentration of potassium, sodium, chloride, calcium in EPS and urine of 31 controls and 79 patients with prostatitis were measured and analyzed. There was no significant difference in the concentrations of potassium, sodium, chloride and calcium between the patients and the controls. Among the patients treated effectively, potassium concentration was 40.66 +/- 17.10 mmol/l before treatment and 33.42 +/- 17.27 mmol/l after treatment. While among the patients treated ineffectively, potassium concentration was measured as 37.57 +/- 16.93 mmol/l and 50.66 +/- 18.77 mmol/l before and after treatment respectively. The concentrations of electrolytes in prostatic fluid varied greatly between individuals. Potassium concentration in EPS decreased significantly after treatment among the patients with obvious treatment effectiveness, while increased among those who failed the treatment. EPS potassium concentration was also found to be lower in patients with pain than those without pain. No significant difference was found between the normal group and the no-pain patients.