A combined supplement of magnesium oxide (300 mg/day) and pyridoxine.HCl (10 mg/day) was given p.o. to 16 recurrent calcium oxalate (CaOx) stone formers, and its therapeutic efficacy was biochemically evaluated by measuring various parameters of blood (Na, K, Mg, urea, creatinine, calcium, phosphate, uric acid, alanine transaminase, aspartate transaminase and alkaline phosphatase) and urine (volume, pH, creatinine, Na, K, Mg, uric acid, calcium, phosphate, oxalate and citrate) at 0, 30, 60, 90 and 120 days of treatment. Serum Mg significantly (P < 0.01) increased after 30 days of treatment and remained constant thereafter while other blood parameters were unaltered. Combined treatment led to a significant increase in the urinary excretion of Mg and citrate over pretreatment values while oxalate excretion showed a gradual and significant decline during the therapy. The results confirmed the efficacy of MgO-pyridoxine supplementation in terms of changes in urinary excretion of lithogenic and inhibitory components, leading to a significant (P < 0.01) decrease in CaOx risk index from 0.09 +/- 0.04 at 0 day to 0.05 +/- 0.02 after 120 days of treatment.