Macrocephaly, a key feature of M-CM syndrome, is typically noted at birth and can be progressive. The macrocephaly may be associated with several neurologic structural anomalies, but the severity of structural brain abnormalities may not correlate with the degree of macrocephaly.23 Similarly, the extent of the PWS does not correlate with the severity or number of associated anomalies. Among the described structural abnormalities are cerebral asymmetry, ventriculomegaly (usually nonobstructive), Chiari type I malformation, acquired cerebellar tonsillar herniation, cortical dysplasia, and polymicrogyria.2,7,8,10,11,13,20 Conway et al11 recently reviewed the neurologic and imaging findings of M-CM in depth and emphasized the potentially progressive nature of central nervous system disease. Clinical signs that should cause concern are a rapidly enlarging head; acute or progressive paresis; focal neurologic signs; seizures; apnea, swallowing problems, oculomotor difficulties, and other brainstem signs; or lethargy, irritability, headache, or other symptoms of increased intracranial pressure. Conway et al11 recommended baseline brain magnetic resonance imaging scans at the time of diagnosis with follow-up studies every 6 months until age 2 years with an interval assessment at 3 years. Developmental delay in M-CM is usually mild to moderate, but early intervention with speech or physical therapy may be helpful. Although cardiac arrhythmias are rare in patients with M-CM, a baseline electrocardiograph is appropriate. Serial limb length measurements are also necessary to address the orthopedic aspects of the condition. Because the vascular stains often fade, decisions regarding laser therapy should be individualized based on their location, extent, and patient and parental preferences.