Immunosuppression management in post-transplant malignancy is challenging because of a lack of objective immunologic assessment tools. The ImmuKnow assay measures the ATP level from CD4 T cells, quantifying cell-mediated immunity and providing an insight into the immune status of transplant recipients. Its potential use in patients with post-transplant de novo malignancy was evaluated. Thirteen adult transplant patients with de novo malignancy were divided into survivors (n = 9) and non-survivors (n = 4) after malignancy treatment. Tacrolimus and the ImmuKnow levels were monitored before, during, and after malignancy treatment. The ImmuKnow level in non-survivors group was significantly lower before and after malignancy treatment compared to survivors group (p = 0.013 and 0.0014 respectively). In survivor group, the ImmuKnow level was significantly decreased during malignancy treatment (p = 0.019) but recovered to the initial level after the treatment. However, in non-survivor group, the ImmuKnow level remained suppressed throughout the observed period despite a reduction in immunosuppressive drug levels. The ImmuKnow assay can be an objective means evaluating immune status of patients with de novo malignancy. The ImmuKnow assay can express the degree of immune suppression induced by chemotherapeutic or radiation therapy and may be a useful tool in optimizing the timing of re-introduction of immunosuppression after malignancy treatment.