BACKGROUND: Dolichoectasia (elongation, dilatation and tortuosity) of the basilar artery can cause an isolated cranial neuropathy. The trigeminal nerve and facial nerve are most frequently affected. Dysfunction of one of the ocular motor cranial nerves due to basilar artery dolichoectasia is uncommon, and an isolated IVth (trochlear) nerve palsy has not been previously described in the literature. HISTORY AND SIGNS: Two men, ages 70 and 59 years, respectively, presented with vertical diplopia due to a IVth nerve palsy. In one patient, the onset of the IVth nerve palsy was painless and gradual and in the other patient, the onset was acute and associated with periorbital pain. Neuroimaging in both patients revealed pathological tortuosity of the basilar artery around the midbrain and displacement of the artery toward the side of the affected trochlear nerve. THERAPY AND OUTCOME: The patients were observed clinically. One patient had gradual worsening of his palsy for three and one-half years then suffered a stroke. The second patient whose IVth nerve palsy had an acute onset experienced spontaneous resolution of his palsy but later developed dysfunction of other cranial nerves. CONCLUSIONS: Basilar artery dolichoectasia should be considered in the differential diagnosis of an isolated IVth nerve palsy. The clinical course may be variable, and the prognosis is not always benign.