Two patients with Lhermitte-Duclos disease were evaluated by brain positron emission tomography (PET) and technetium-99m-ethyl cysteinate dimer (Tc-99m-ECD) single-photon emission computed tomography (SPECT). In the lesions in both patients, hyperperfusion was detected on cerebral blood flow images obtained by PET, and hyperactivity by standard Tc-99m-ECD SPECT. Dynamic Tc-99m-ECD SPECT images demonstrated a plateau of activity in each lesion. These findings suggest that lesions in Lhermitte-Duclos disease have a retention mechanism for Tc-99m-ECD equivalent to that of normal neural tissue.