Subdural cerebrospinal fluid collection and ventricular dilatation occur rarely after foramen magnum decompression. We report 2 cases of symptomatic subdural fluid collection after uncomplicated foramen magnum decompression for Chiari I malformation with syringomyelia. In these cases, postoperative magnetic resonance imaging showed significant shrinkage of the syrinx. The patients subsequently experienced clinical deterioration on days 9 and 13 after discharge, respectively. Repeat magnetic resonance imaging showed bilateral infratentorial subdural fluid collection extending supratentorially, passing through the tentorial notch, and with prominent midline shift. Inferior decent of the cerebellum into the decompression field was also demonstrated. Following evacuation of the subdural fluid by burr-hole and drainage for 3 days, clinical signs did not improve until symptomatic treatment with strict bed rest in the Trendelenburg position was prescribed. In such cases the arachnoid either should not be opened, or should be widely opened and expansile duraplasty performed. It is the authors’ opinion that the latter is the appropriate treatment, followed by conservative therapy including the Trendelenburg position, and restriction of ambulation and effort.