A daughter of first cousins had two extremely rare, recessive disorders: thrombasthenia (Glanzmann's disease, glycoprotein IIb/IIIa deficiency) and mucopolysaccharidosis IIIB, (Sanfilippo B syndrome, alpha-N-acetylglucosaminidase (NAG) deficiency). Normal alpha-N-acetylglucosaminidase activity was observed in two obligate heterozygotes (the proband's father and her maternal grandmother), suggesting that in addition to the normal and defective alleles, a third, hyperactive allele is also present in this family. Such a hyperactive allele seems to be quite prevalent in our area, and makes the biochemical identification of heterozygotes impossible if no extensive family surveys provide additional clues. There was no linkage between the two diseases, nor between any of them and several blood-groups and HLA-antigens tested for.