MR examinations were performed on 16 patients with small (< 35 mm diameter) solid renal tumors previously diagnosed with US and/or CT. The study was aimed at assessing MR accuracy in the identification and characterization of this type of lesion. The studies were performed with a 0.5 T GE MR Max Plus unit, with no paramagnetic contrast medium administration. MRI detected the lesion in 15 cases (93.7%). In 3/15 patients (18.7%) the tumors had signal patterns similar to those of simple renal cysts--i.e., homogeneously hypointense on T1 and markedly hyperintense on T2-weighted images. In one patient (6.2%) MRI failed to detect the lesion. In the extant 12 cases the tumors were correctly demonstrated but had different signal patterns. On T1-weighted images 6 lesions were hypointense, 2 hyperintense because of hemorrhagic content, and 4 exhibited poor contrast with the surrounding renal parenchyma. On T2-weighted images 4 lesions were inhomogeneously hyperintense, 3 were hypointense because of fibrosis or old blood content, and in 5 cases contrast between the tumor and the renal parenchyma was poor. The authors conclude that T1-weighted MR images, in a midfield MR unit, are more sensitive than T2-weighted images in identifying small renal tumors. MRI is less accurate in the evaluation of these lesions than CT and US. In particular, since small renal lesions may be misdiagnosed as simple cysts, the use of other imaging modalities--e.g., US and CT--is recommended if a renal cyst is first identified on MR images.