Our objective was to assess and compare the diagnostic sensitivity of conventional MRI (cMRI), magnetization transfer imaging (MTI), diffusion-weighted imaging (DWI), and proton magnetic resonance spectroscopic imaging ((1)H-MRSI) in patients with amyotrophic lateral sclerosis (ALS) and primary lateral sclerosis (PLS). Thirty-eight ALS patients, nine PLS patients, and 22 healthy controls were enrolled. cMRI, MTI, DWI and (1)H-MRSI were obtained. ALS patients were classified as advanced phase (Ap)-ALS (definite+probable) and early phase (Ep)-ALS (possible+probable-laboratory supported). cMRI was highly sensitive in detecting corticospinal tract (CST) hyperintensities in Ap-ALS (63.4%) and PLS (71.9%), but it was poorly sensitive in Ep-ALS (17.1%). Hyperintensity on proton density-weighted images correlated with ALS severity (p=0.02). CST apparent diffusion coefficient was significantly increased in ALS (p<0.01) and PLS (p=0.02) versus controls. The N-acetylaspartate/creatine ratio was significantly reduced in the motor cortex of patients versus controls (p< or = 0.01 in PLS, p=0.02 in Ap-ALS). The study shows the utility of cMRI for diagnosing ALS. Nevertheless, MRI sensitivity is limited at the early stages of the disease. In these cases, DWI and (1)H-MRSI seem to have the potential to ameliorate the patients' work-up and estimate the nature and extent of the underlying pathological damage.