AbstractChorionic villus sampling was performed on two patients with a previous family history of Sandhoff's disease. Total β‐hexosaminidase (Hex) activity in case 1 was within the normal range (case 1: 6365 μmol/h/g protein; control range: 3227‐24 495/miol/h/g protein). The β‐hexosaminidase isoenzyme pattern was found to be normal. These results were confirmed on cultured amniotic fluid cells. In case 2, the total Hex activity was 672 μmol/h/g protein, i.e., 7 per cent of the control mean (10 085 μmol/h/g protein), and chromatography demonstrated that more than 50 per cent of this activity was due to the abnormal isoenzyme β‐hexosaminidase S (Hex S). The fetus was predicted to be affected by Sandhoff's disease and this was confirmed on fetal tissues after termination of pregnancy. This study demonstrates that a fetus affected by Sandhof's disease can be reliably diagnosed during the first trimester of pregnancy.