Intervention fidelity refers to strategies that practitioners and researchers use to monitor, enhance, or evaluate the accuracy and consistency of the delivery of an intervention to ensure that it is implemented according to how it was planned. The purpose of the authors in this article was to evaluate intervention fidelity in the psychosocial oncology intervention effectiveness research. Twenty-eight studies located in a previous systematic review on psychosocial oncology intervention effectiveness comprised the sample for this research. A treatment fidelity checklist was applied to each study independently by each author (MP & PB). Percent agreement between raters ranged from 68% to 100% (M = 89%). Overall, the mean proportion of adherence was 0.57 (SD 0.12), which may be considered to be moderate fidelity. Critical examination and applicability of the checklist in examining and assessing intervention fidelity were highlighted and discussed. Overall, intervention fidelity was adequately addressed in the psychosocial oncology intervention effectiveness research, and integrity was confirmed in the majority of studies reviewed. Suggestions for future psychosocial oncology effectiveness research were made.