Eighteen cases affected by Sandhoff disease were investigated by an enzymatic study of serum and leukocytes during the period 1988-1996, the clinical expression and enzymatic study were reported and discussed, An indirect minimum disease incidence was calculated in the Turkish population. Hexosaminidase activity in serum and leukocytes was severely deficient when measured by synthetic substrate 4-MU-N-acetylglucosaminide using the thermolabile fractionation procedure, Fractionation of hexosaminidase revealed different levels of isoenzymes A and B. Clinically, organomegaly was not found in 11 out of 18 infantile Sandhoff disease patients, while the remaining seven had mild organomegaly. Organomegaly was not found in patients with relatively high % hexosaminidase B activities. These results suggested that patients with different percent heat-stable enzyme activity may have a different type of mutation which is related to the underlying molecular heterogeneity in the Turkish population where 21% of marriages are found to be consanguineous. (C) 1997 Elsevier Science B.V.