Magnetic resonance imaging (MRI) is the best method for assessing myelination in infants and young children. Although delayed myelination is a common neuroradiologic diagnosis, there are few or no data regarding the reliability of this diagnosis or radiographic and clinical findings in cohorts of such patients. We evaluated the cranial MRI scans of 109 patients from age 0 to 36 months, without knowledge of any patient's age or previous clinical or radiologic diagnosis. For each cranial MRI, seven neuroradiologic landmarks were evaluated and established criteria used to assess the state of myelination. We found that in 12 of 109 patients, delayed myelination was misdiagnosed, whereas the diagnosis of delayed myelination was missed in four other patients. Lack of familiarity with the myelination milestones of infancy was the most common reason for a misdiagnosis of delayed myelination. Failure to recognize delayed myelination was due to a failure to appreciate the forceps minor as a landmark. Overall, the diagnosis of delayed myelination was inaccurately applied or missed in 15% of the patients in this series. Of the 14 patients identified as having delayed myelination, 10 had other central nervous system structural abnormalities seen on MRI, most commonly cortical atrophy. Developmental delay was the most common clinical correlate of delayed myelination and was documented in 12 of the 14 patients. To increase the reliability of neuroradiologic assessments in young children, we propose that central nervous system myelin maturation be evaluated and expressed as a myelination age equivalent, analogous to the assessment of pediatric bone age using conventional radiographs.(ABSTRACT TRUNCATED AT 250 WORDS)