Normal urine contains small amounts of glucose, called basal glucosuria, and other carbohydrates. Increased amounts of glucose beyond the basal excretion rates i.e. frank glucosuria, reflect reduced activity of tubular glucose reabsorption. Clinically, there are two conditions which are known to appear with a primary disturbance of epithelial glucose transport: intestinal glucose-galactose malabsorption and benign familial renal glucosuria. In the latter, both the renal threshold for glucose and maximal tubular glucose reabsorption are diminished. The degree of glucosuria is variable; the most severe defect demonstrates minimal glucose threshold values and extremely low levels of maximal glucose reabsorption (type 0). The moderate and mild types show variable reductions of both functional parameters. It is questionable whether these should be subdivided into type A and type B glucosurias. Data in the literature reveal that two distinct entities do not exist, but that there is instead a continuous transition from low to normal values. The defect is transmitted genetically, either in an autosomal dominant or autosomal recessive manner. It seems plausible that the different defects might be produced either by mutations on different gene loci, or by multiple alleles of the same gene locus that determines the synthesis of the glucose carrier.