Inflammation provides favourable microenvironment for cancer development. An enhanced COX-2 gene expression is a key inflammatory mediator of cancers and the drug that inhibits it, helps to manage cancer effectively and increases survival rate. The objective is to analyse the inflammatory changes and COX-2 gene expression in benzo (a) pyrene induced mice and to evaluate the regulatory effect of all trans retinoic acid. The body and organ weights were recorded in B(a)P induced mice. The haematological parameters and serum inflammatory markers of carcinogenesis were tested. The H & E stained liver and lung tissues were examined for histopathologic changes. The COX-2 gene expression was analysed by RT-PCR and qPCR in lung and liver. The decreased body weight, increased organ weights and the damages in liver and lung were observed in B(a)P induced mice and were prevented significantly upon ATRA treatment. The lowered Hb, RBC and lymphocytes and an enhanced WBC, monocytes and neutrophils observed in B(a)P group were significantly reversed in treated group. A drastic increase in cancer associated inflammatory markers observed in B(a)P induced mice were significantly (P ≤ 0.001) reduced in treated mice. The RT-PCR product density of COX-2 gene was very high in B(a)P group (lung-0.43 ± 0.06; liver-0.39 ± 0.04) significantly lower in treated group (lung-0.12 ± 0.03; liver-0.08 ± 0.03) with a significant difference in RQ values (B(a)P lung-18.46 ± 0.04, liver-12.46 ± 0.08; treated lung-5.93 ± 0.07, liver-2.92 ± 0.10). The ATRA has decreased the inflammatory condition with downregulation of COX-2 gene expression and thereby prevented carcinogenesis during early stage of B(a)P induced cancer development. • Association between inflammation and lung cancer development has been proven in benzo(a)pyrene induced cancer model • Simultaneous ATRA treatment has significantly reduced the cancer inflammatory markers in benzo(a)pyrene induced mice. • Hematological abnormalities in benzo(a)pyrene induced cancer model were significantly minimized by ATRA treatment. • Abnormalities in the lung/liver weights and histology in benzo (a) pyrene induction were reduced by ATRA treatment. • ATRA treatment has decreased the expression of COX2 gene in benzo(a)pyrene induced mice as observed in qPCR result. [ABSTRACT FROM AUTHOR]