Orthostatic headache related to spontaneous cerebrospinal fluid leak (CSF) appears within 2 h of sitting or standing in most patients. However, longer delays to headache onset have been observed, including some patients who have headaches only in the afternoon. The objective of this study is to describe second-half-of-the-day headache as a manifestation of spontaneous CSF leak and propose potential mechanisms. From 142 patients evaluated by one of us (B.M.) during a 10-year period for spontaneous intracranial hypotension, those describing headache occurring exclusively in the afternoon accompanied by typical changes of intracranial hypotension on head MRI were retrospectively identified and their medical records reviewed. Five patients met our pre-defined inclusion criteria (5/142, 3.5%; three women; mean age 50 years). Second-half-of-the-day headache was an initial symptom of intracranial hypotension in one patient, spontaneously evolved from prior all-day orthostatic headache in one patient, and was a residual or recurrent symptom after epidural blood patch in three patients. Head MRI changes due to intracranial hypotension were decreased during second-half-of-the-day-headache compared to typical all-day orthostatic headache in three out of four patients. The timing of second-half-of-the-day headache and orthostatic headache in the clinical course of patients with spontaneous CSF leaks and related MRI findings suggest that second-half-of-the-day headache is likely a manifestation of a slowed or slow-flow CSF leak.