To investigate eosinophilia as a potential on-treatment biomarker for patients receiving cancer immunotherapy.

We evaluated the association between eosinophilia and treatment response and toxicity in a retrospective cohort of patients receiving cancer immunotherapy.

The study involved 146 patients. Eosinophilia developed in 22%. Patients who developed eosinophilia were more likely to achieve disease control (p = 0.009), with every 0.1 × 10/l rise in eosinophil count, while receiving treatment was associated with a 28% relative increased chance achieving disease control. Although there was a trend toward improved survival, there was no significant association between eosinophilia and improved overall survival (p = 0.136). Patients with eosinophilia were more likely to develop toxicity (p = 0.042).

Eosinophilia is a potentially useful biomarker warranting further prospective clinical investigation.