<jats:p>Progressive muscle weakness of at least 3 years' duration is described in a 6-year-old Caucasian girl. Neurological abnormalities included loss of muscle bulk without fasciculation, muscle weakness, areflexia, and impairment in perceiving touch, position sense, and vibration. The cerebrospinal fluid was normal. A clinical diagnosis of polyneuropathy was made and electrophysiological studies were consistent with that diagnosis.</jats:p> <jats:p>No gross abnormality of a left sural nerve biopsy was detected, but light microscopic study of transverse paraffin sections revealed abnormally large argentophilic masses scattered through each fascicle Examination of 2 microns-thick plastic embedded sections showed the argentophilic masses were axons which were segmentally enlarged to enormous proportions.</jats:p> <jats:p>Ultrastructurally, axons were distended by masses of tightly woven neurofilaments. Unusual features included the tendency to occur in massive whorls and the frequent occurrence of electron dense condensations into which parallel skeins of neurofilaments appeared to gather. The nature of the electron dense condensations could not be further resolved.</jats:p> <jats:p>Axonal collections of neurofilamentous masses have been described in experimental neuropathies due to acrylamide, mitotic spindle inhibitors and in some cases of motor neurone disease. However, the accumulation of neurofilamentous masses in these disorders does not approach the proportions encountered in this case.</jats:p>