We monitored the serial changes of natural killer cell (NK) activity in eight recipients of living‐related liver transplantation. The HLA types of all eight patients were haplotypically identical with those of their donors. Tacrolimus and methylprednisolone were used for immunosuppression. The NK activity before transplantation was 24.1 ± 20.2 % which is surprisingly low when compared with the value for normal individuals (67.7 ± 13.2%, P < 0.01) or a liver dysfunction group (49.4 ± 21.9%, P < 0.05). Serial changes in NK activity revealed a minimum of 6.1 ± 3.6% 1 week after transplantation, gradually increasing to 49.2 ± 12.5 % at 2 months after transplantation. These results suggest that the diseased liver might play an important role in the suppression of NK activity.