There is immense variability in the clinical presentation of tuberous sclerosis and many incomplete forms (formes frustes) exist. To investigate the imaging characteristics of cortical tubers seen in tuberous sclerosis unaccompanied by other stigmata, we reviewed MRI and CT of six patients who met the criteria for a definitive diagnosis of TS, established solely by the presence of a histologically confirmed cortical tuber. Five of the patients had a solitary cortical tuber and the last had three lesions, one of which was resected and confirmed histologically. The other two lesions were included in our study. CT showed five tubers as low density, but three were not identified. No calcified or dense lesions were observed. MRI revealed peripheral components and inner cores of seven cortical tubers in five patients, with differing signal characteristics. The subcortical cores, with T1 and T2 prolongation, were separated from the overlying cortex. Abnormal inhomogeneous high signal was observed in both the cortex and subcortical white matter on proton-density weighted or FLAIR images. A radially orientated white-matter band was observed in one patient, and central depression of the expanded gyri in another. In one patient, a cortical tuber was atypical, with a thick cortex on T1-weighted images and a blurred grey/white matter junction with diffusely increased signal on T2-weighted images. Cortical tubers without other stigmata of tuberous sclerosis are shown to be distinct from focal cortical dysplasia.