Objective To invesitigate the application of contrast enhanced ultrasound in differentiating focal organizing pneumonia( FOP) and primary lung cancer . Methods The imaging data of 23 cases with FOP ( FOP group) and 75 cases with primary lung cancer ( primary lung cancer group) on conventional ultrasound and contrast enhanced ultrasound were retrospectively analyzed . The size and arrival time( AT) of the contrast agent and the enhanced pattern of the two groups were compared . ROC curve was created to determine the most accurate AT for differential diagnosis . Results There was no significant difference in the gender and the size of nodule between the two groups ( P > 0 .05) . The age of FOP group was younger than that of primary lung cancer group ( P = 0 .013) . The AT of FOP group was much earlier than that of primary lung cancer group [ ( 6 .9 ± 2 .4) s vs ( 11 .4 ± 4 .3) s , P = 0 .000] . In FOP group ,20 patients ( 87 .0％ ) showed centrifugal enhancement , 2 patients ( 8 . 7％ ) showed centripetal enhancement and 1 patient ( 4 .3％ ) showed diffuse homogeneous enhancement ,respectively .In primary lung cancer group , 12 patients ( 16 .0％ ) showed centrifugal enhancement , 58 patients ( 77 .3％ ) showed centripetal enhancement ,2 patients ( 2 .7％ ) showed diffuse homogeneous enhancement and 3 patients ( 4 .0％ ) showed diffuse heterogeneous enhancement ,respectively . There was significant difference in the enhanced pattern between the two groups ( P = 0 .000) . Meanwhile ,8 patients in FOP group ( 34 .8％ ) and 31 patients in primary lung cancer group ( 41 .3％ ) had unenhanced region in the nodule ( P = 0 .574) . ROC analysis demonstrated that AT of 8 .5 s was the best cut-off value for the differential diagnosis . When AT earlier than 8 .5 was taken as diagnostic criterion for FOP ,the diagnositc sensitivity ,specificity were 74 .7％and 82 .6％ ,respectively . Conclusions Contrast-enhanced ultrasound can provide evidence in differentiating FOP from primary lung cancer .