Abstract: AimTo study the effects of 9-cis-retinoic acid (9-cis-RA) on cell cycle and expression of cyclin D1 and cdk4 in lung cancer cells. Methods9-cis-RA (1×10-6 mol·L^-1) was used to treat lung cancer cells for 24 h; Flow cytometry (FCM) was used to detect the percent of G₀/G₁ phase and S phase cells of three groups including blank control, DMSO control and 9-cis-RA groups; RT-PCR was used to analyze the expression changes of cyclin D1 and cdk4 before and after treatment with 9-cis-RA in lung cancer cells. Results The percent of G₀/G₁ phase cells of 9-cis-RA groups was significantly higher than that of the control groups (P<0.01 or P<0.05) and the percent of S phase cells of 9-cis-RA groups was lower than that of the control groups (P<0.01 or P<0.05); the expression of cyclin D1 of PG, SPC-A1 and L78 cells was decreased (P<0.01) and the expression of cdk4 of PG, A549 and L78 cells was also decreased (P<0.01) after treatment with 9-cis-RA. ConclusionMost of the proliferation and the expression of cyclin D1 and cdk4 of PG, A549, SPC-A1 and L78 were inhibited by 9-cis-RA.