Renal function studies were performed in 41 patients with sickle cell-β thalassaemia (S/b thai) and compared to 14 normal controls and 8 sickle cell (SS) patients. Polyuria, hyposthenuria and mild proteinuria were common in both S/b thai and SS patients. A renal concentrating defect was manifest in all patients studied, and in 4 of the 7 S/b thai patients tested, an abnormal acidification test was found. A statistically significant negative correlation (n=19, r = -0.48, p < 0.05) was noted between creatinine clearance (CCr) and age for the patients over 30 years. There was no correlation between hemoglobin and CCr; on the contrary, a statistically significant negative correlation was found between CCr and hemoglobin F (n = 29, r=-0.428, p < 0.05) Our S/b thai and SS patients showed a decreased daily excretion of sodium, calcium, phosphate and magnesium and lower serum magnesium levels than the controls. One third of the S/b thai patients showed hyperuricosuria, and a statistically significant negative correlation was noted between serum uric acid and its fractional excretion in all S/b thai patients (n = 41, r = -0.450, p < 0.01). Serum phosphate levels were independent of age. A statistically significant positive correlation was found between the tubular reabsorptive capacity for phosphate and the number of painful crises per year (n = 33, r = 0.836, p < 0.001). We conclude that renal involvement in the double heterozygous state is as severe as in homozygous sickle cell disease