OBJECTIVE To determine the magnetic resonance (MR) imaging characteristics of amyotrophic lateral sclerosis (ALS) and to evaluate possible correlations between the disease severity and the MR imaging findings. METHODS The authors retrospectively reviewed MR images of the head in 17 patients with ALS (14 men and three women) and 17 age-matched control subjects. The corticospinal tract (CST) was analyzed for the following features: size, signal intensity, and number of sections and regions in which the CST could be seen. RESULTS Patients with ALS demonstrated sharp, well-defined, round, symmetric lesions that were hyperintense to gray matter within the CST. The lesions were best seen at the level of the middle or lower internal capsule on T2-weighted images. Visualization of the CST on proton-density-weighted images (which occurred in eight ALS patients) is the most reliable MR finding. Low signal intensity was identified within the motor cortex in six patients. Positive MR findings correlated with average or rapid progression of the disease. CONCLUSIONS Increased signal intensity in the CST on proton-density-weighted MR images is diagnostic for motor neuron disease and correlates with the rate of disease progression. Subtle ALS changes can be differentiated from the normal CST. Low signal intensity in the motor cortex on T2-weighted images is a useful finding.