<h4>Background</h4>Granulocyte-colony stimulating factors (G-CSFs) are widely used to prevent chemotherapy-induced neutropenia, but they have been linked to coronary neovascularization and prothrombotic effects.<h4>Case report</h4>A 62-year-old man with small cell lung cancer developed an acute posterior ST-segment elevation myocardial infarction (STEMI) 1 day after receiving 6 mg of pegfilgrastim during his second chemo-immunotherapy cycle. Here, CSF3 is linked to Decreased total neutrophil count.