Fifty patients with intramedullary space occupying lesions were studied with MR from October 1983. Of these 32 were operated on and histological diagnosis together with gross morphological description was available. MR findings were reviewed in search for morphological and signal characteristics capable of providing a better preoperative diagnosis. There were 11 astrocytomas, 10 ependymomas, 5 vascular lesions and 6 "gliomas". All lesions usually had signal characteristics different from those of the normal cord, mainly because of prolonged T1 and T2 relaxation times. No specific patterns related either to morphology or signal were found, capable of differentiating the two main histological types. Vascular malformations may be suspected from the presence of serpiginous areas without signal. Tumor nodules within cysts can however be well recognized, thus providing the neurosurgeon with very useful preoperative information. Some characteristics of signal and morphology, are more common for one or the other of the two main groups of tumors. A tumor involving all the cord with a homogeneously prolonged T2 relaxation time is most likely to be an astrocytoma. A small nodule with signal close to normal parenchyma, associated with extensive cysts, is most likely to be an ependymoma.