Primary renal glucosuria is an inherited defect of tubular glucose reabsorption and usually classified in type A and type B. We now observed a new type in a 15-year-old boy who had a complete absence of tubular glucose reabsorption. His father had a daily glucosuria of 1.1 g/1.73 m2 and his mother of 2.7 g/1.73 m2. Two siblings excreted 0.4 g/1.73 m2 and 0.3 g/1.73 m2 glucose and one sister had no glucosuria. The proband excreted daily 136 to 160 g/1.73 m2 glucose accompanied by normal blood glucose levels between 75-105 mg/dl. The glomerular filtration rate (inulin clearance) was 148-153 ml/min/1.73 m2 and the endogenous glucose clearance was 112-160 ml/min/1.73 m2 when blood glucose levels were 72-82 mg/dl. Thus, glucose clearance was nearly identical to inulin-clearance. After intravenous glucose loading with a blood glucose concentration of 261-342 mg/dl, glucose clearance remained in the same range and tubular glucose reabsorption was virtually absent. There were no disturbances in tubular reabsorption of other substrates. This new type of primary renal glucosuria was not recognized thus far, and we propose to call it type O glucosuria. The family tree revealed consanguinity and most probably the proband is homozygous and both his parents are heterozygous for type O renal glucosuria.