Generally, the data obtained in this study showed that 5-FU, an important chemotherapeutic agent, causes lung problems such as hyperplasia, cell infiltration and pneumonia in addition to developmental disorders, disrupts the oxidant-antioxidant balance in favour of oxidants, triggers inflammation by increasing NFκB and IL-1β levels, and stimulates apoptosis by increasing Bax and caspase-3 levels and decreasing Bcl-2 levels. The gene discussed is CASP3; the disease is pneumonia.