The relationship of protein intake to urinary oxalate and glycolate excretion was examined in a large cohort (N = 101) of normal individuals on self-selected diets and in 11 individuals on controlled protein diets. On self-selected diets no correlation was detected between protein intake and urinary oxalate or glycolate excretion. A moderate but significant correlation (r = 0.45; P < 0.001) of oxalate with urea excretion was observed in males but not females, suggesting that there may be a link between urea and oxalate synthesis in males. On controlled protein diets mean oxalate excretion in females on days 7 to 10 of a high protein diet (1.8 g/kg body wt) was 20% higher than on a low protein diet (0.6 g/kg body wt; P = 0.02), but there was no difference in males. Glycolate excretion was significantly higher (P < 0.001) on the high protein diet than on the low protein diet in both sexes. Only a weak precursor-product relationship was observed between glycolate and oxalate. A gender effect was apparent on both self-selected and control diets with females excreting more oxalate and glycolate relative to creatinine than males. A pronounced inter- and intra-individual variability in the excretion of oxalate was observed, even on controlled diets. This suggests that genetic factors and physiological changes such as hormonal fluctuations may contribute more to the variability in oxalate excretion than the dietary intake of protein.