Urinary oxalate excretion has been measured by a specific enzymatic method in normal subjects and stone formers with idiopathic hypercalciuria. In every group studied urinary oxalate was higher in the summer than in the winter. These differences were slight and not significant in normal subjects but were considerable and statistically significant in the stone formers both untreated and when treated with thiazide. Thiazides raise urinary oxalate only very slightly but cellulose phosphate leads to large rises in urinary oxalate both in the summer and the winter. The highest values of urinary oxalate were seen in the summer in patients treated with cellulose phosphate. The mean rise in this group was 70% above normal and this must be viewed with some anxiety.