OBJECTIVE. We performed this study to evaluate the incidence and significance of ring enhancement after IV administration of an ultrasmall superparamagnetic iron oxide (USPIO) particle (Code 7227), a reticuloendothelial contrast agent with potential use as a blood-pool agent, for characterizing focal hepatic lesions. SUBJECTS AND METHODS. Conventional T1-weighted imaging, fat-suppressed T1-weighted imaging, conventional TS-weighted imaging, and fast TP-weighted imaging of the liver were obtained in 27 patients with 43 liver lesions before and after IV administration of a USPIO preparation. All lesions were larger than 1 cm; 29 were malignant and 14 were benign, Diagnosis was confirmed in all cases, either pathologically (19 patients) or by follow-up examination (eight patients). Two readers independently evaluated each pulse sequence for the presence of ring enhancement of hepatic lesions. RESULTS. Ring enhancement was noted only on T1-weighted images, with no ring enhancement evident on T2-weighted images, Twenty of 43 (47%) lesions showed ring enhancement, including 18 of 29 (62%) malignant lesions and two of 14(14%) benign lesions (p < .011; Wilcoxon signed rank test). Fat-suppressed T1-weighted imaging showed ring enhancement better than or equal to conventional T1-weighted imaging in all cases, with ring enhancement of 15 of 18(83%) malignant lesions and two of two benign lesions better demonstrated on fat-suppressed T1-weighted imaging sequences (p less than or equal to .025). CONCLUSION. Ring enhancement after IV administration of Code 7227 is a frequent finding seen more often with malignant than benign lesions, potentially identifying a new MR imaging feature for the characterization of liver lesions. The identification of ring enhancement on T1-weighted images attests to the significant blood-pool effects of USPIO particles.