The objective of this study was to determine the frequency of electrolyte perturbations and their relationship with leukemic status before and after chemotherapy in patients with acute lymphocytic leukemia. Blood biochemistry, liver and renal function tests, and renal sonograms were examined at diagnosis and during induction therapy in 334 patients. Renal and electrolyte disturbances were then studied in 116 patients between 3 and I 10 months after cessation of the St. Jude chemotherapy treatment protocol. Glomerular filtration rate, electrolyte, protein, and beta-2-microglobulin levels were determined in fresh urine samples, and serum electrolyte levels were examined in blood samples. Renal sonographic examinations and scintigraphic examinations were performed with DMSA and MAG-3. Renal leukemic involvement was detected by sonographic examination in 32 patients who had also presented with hyperphosphatemia or hyperuricemia. Patients with electrolyte disorders at diagnosis were less likely to have tumor lysis syndrome during induction chemotherapy. This may be explained by correction of their electrolyte disorders at the time of diagnosis, which may protect them from tumor lysis syndrome. Hypocalcemia and hyponatremia at the time of diagnosis were found to be significant initial risk factors for renal scan abnormalities and microproteinuria, respectively, during the late therapy period (P < 0.05). Electrolyte abnormalities and renal changes were commonly observed before and after therapy for leukemia. Patients presenting with hypocalcemia and hyponatremia should be examined for microproteinuria and should undergo renal scanning during the late therapy period.