The purpose of the present study was to determine how dietary flaxseed oil altered specific biomarkers in its protection against bleomycin‐induced pulmonary fibrosis. Twenty weanling rats were randomly assigned to one of two groups: (1) a group fed a 15% (w/w) corn oil (control) diet and (2) a group fed a 15% flaxseed oil diet. Rats were fed their respective diets for 4 weeks prior to bleomycin treatment. Half of each group received either bleomycin or saline vehicle via oropharyngeal delivery. All rats were euthanized at day 7 post‐treatment. Formalin‐treated lung tissue were immunohistostained using antibodies for proliferating cell nuclear antigen (PCNA), Cux1 (CUTL1), and cyclin‐dependent kinase inhibitor 1B (p27). We found that PCNA was overly expressed in bleomycin‐induced rat lungs but was notably reduced in rats given the flaxseed oil‐supplemented diet. Expression of p27 was moderately reduced in rats given the flaxseed oil‐supplemented diet as compared with groups given corn oil. No significant differences were found for Cux1. Results indicate that PCNA may be another biomarker for pulmonary fibrosis and its decreased expression suggests that the dietary flaxseed oil inhibits proliferation of bleomycin‐treated lung cells.