A man with a subclinical cobalamin deficiency developed syncope, vertigo, paresthesias, and ataxia after two exposures to nitrous oxide anesthesia. Patients with unrecognized cobalamin deficiency may be particularly susceptible to brief exposures to nitrous oxide, which inactivates the cobalamin-dependent enzyme methionine synthase and may cause a myeloneuropathy. Clinicians should consider this entity when confronted with patients with neuropathic symptoms after surgical or dental procedures.