Rapid scan techniques have introduced new sequence parameters as well as novel contrast concepts into everyday magnetic resonance imaging (MRI). In particular contrast characteristics of fast-spin echo (FSE) sequences showed some significant differences when compared to conventional spin echo images. The purpose of this work was to demonstrate the capabilities of FSE MRI in identifying and characterizing the in vivo anatomy of the main cerebral associative systems. Between March and November 1995, 20 healthy adult volunteers (12 males, 8 females, mean age 35 years) were submitted to a cranial MRI examination (1.5 Philips Gyroscan NT). In all cases axial and coronal 2-dimensional FSE T2-weighted and 2-dimensional inversion recovery FSE T1-weighted images were obtained. All MRI images were examined by a neuroradiologist (G. Dal Pozzo) for the depiction of the following compact white matter fiber bundles: anterior commissure, corpus callosum, superior fronto-occipital fasciculus, cingulum, fornix, mammillothalamic tract, uncinate fasciculus, superior and inferior longitudinal fasciculus. All these associative pathways could be well identified on T2-weighted images due to a lower signal intensity with respect to the surrounding white matter. On T1-weighted images only the corpus callosum, the anterior commissure and the fornix could always be identified. Correlation with myelin-specific colorations (Luxol fast blue stains) in anatomic atlases and a review of the literature on the myelinization process during infancy indicate that the short T2 relaxation times of the aforementioned cerebral associative systems may be due to heavy myelination and high fiber density. The correct visualization of interintrahemispheric associative white matter fiber bundles may play an important role in white matter disorders like dys- and demyelinating diseases and in the spreading of vasogenic edema and/or tumor being useful for their staging.