The head impulse test is a simple clinical test comprising high acceleration head rotation. In the presence of a severe unilateral vestibular weakness the normal vestibulo-ocular reflex is replaced by a misalignment of the eye followed by a series of corrective saccades which are evident to the examiner. Previous reports have shown the high sensitivity of the head impulse test in detecting complete unilateral weakness, but indicate poor sensitivity for mild weaknesses. This prospective, blinded study examined the head impulse test in a general clinical population of balance disorder patients to examine the sensitivity and specificity of the test, and to determine the degree of vestibular weakness that is required before the test becomes positive. One hundred and fifty patients were examined and the head impulse test results were compared to results from bithermal caloric testing. Results show that the overall sensitivity of the head impulse test is 34% with a specificity of 100%. The test does not detect mild or moderate vestibular weaknesses but is very sensitive to the presence of a severe paresis (87.5%). Head impulse testing will not replace caloric testing but is a very useful adjunct to it.