To evaluate the effectiveness of CT and MRI at 0.5 T in the diagnosis and staging of retinoblastoma, we studied 11 patients in whom retinoblastoma was clinically suspected. Nine of the eleven had surgically proven retinoblastoma; in the other two a diagnosis of Coats' disease was made. MRI was not as specific as CT for diagnosing retinoblastoma, due to its lack of sensitivity in detecting calcification; it did, however, have superior contrast resolution. On MRI, Coats' disease was reliably diagnosed and easily differentiated from retinoblastoma. Moreover, the greater ability of MRI to differentiate subretinal fluid from tumour also confers high accuracy in measuring tumour size. CT is still the study of choice in the diagnosis of retinoblastoma, but when MRI is available, it should be performed for better differentiation from lesions such as Coats' disease.