Familial dysautonomia (FD) is an inherited disorder that is known to affect both sensory and autonomic functions as a result of incomplete neuronal development and progressive loss but the degree to which patients are affected differs greatly. To determine if quantitative vibration and thermal testing refined the assessment of severity, 23 familial dysautonomia patients were evaluated by clinical examination, measurements of median, peroneal and sural nerve conduction velocities (NCV), and assessment of vibration thresholds at two body sites and of warm and cold perception thresholds at 6 body sites using the method of limits. Data from 80 age-matched normal individuals provided control data for vibration and temperature thresholds. All familial dysautonomia patients had abnormal thermal thresholds. Vibration perception was abnormal in 20 patients. NCVs were slowed in 8 of 16 patients who agreed to be tested. Abnormalities in thermal thresholds are consistent with the reduction of small nerve fibers in familial dysautonomia Abnormal vibration thresholds might be due to disturbed conduction of vibratory impulse trains and reflect the degree to which the disorder is progressive. Vibration and thermal sensation testing were better accepted and provided more information than NCV regarding severity of disease.