The aim of this study was to determine whether rapidly acquired MRI sequences, taking less than 5 min imaging time, can accurately characterise renal masses. All patients found to have a renal space-occupying lesion on CT or asked to participate in a prospective study using rapidly acquired MRI. The MRI technique was performed on a GE Signa (General Electric, Milwaukee, Wis.) 1.5 T magnet using breath-hold coronal and axial T1 GRASS (fast spoiled gradient-recalled acquisition into steady state, FSPGR30/90) and axial T2 fast spin-echo sequences. The results were analysed by two radiologists unaware of the CT or ultrasound findings. The CT/US was independently viewed by a third radiologist. Lesions were characterised as simple cysts, indeterminate or solid. The MR and CT/US results were correlated and the sensitivity and specificity of MR for the characterisation of simple cysts and solid renal masses calculated. A total of 144 lesions (68 patients; 29 women 39 men, age range 32-78 years, average age 60 years) were studied: 111 simple cysts; 3 hyperdense cysts; 26 renal cell carcinomas; and 4 indeterminate lesions on CT/US. There was agreement between the CT/US and MRI in 82% of cases. All renal cell carcinomas were correctly characterised on MRI. Of simple cysts, 79% were correctly identified using this technique. Breath-hold MRI performed in less than 5 min can accurately characterise the majority of renal masses. It is 100% sensitive in the characterisation of renal carcinoma, and it correctly identified approximately 80% of simple cysts. If used at the time a renal mass is detected on MRI, it would reduce the need for further investigation of the majority of incidentally detected lesions which are simple cysts.