Autism is a developmental disability characterized by severe, pervasive deficits in social interaction, communication and range of interests and activities. The neurobiologic basis of autism is well accepted, although the specific etiology is unknown. It has been theorized that autism may result from a combination of predisposing genes and environmental factors. While autism has a known association with environmental factors such as rubella and valproic acid exposure in utero, other proposed environmental mechanisms such as mercury toxicity or other heavy metal exposure have minimal research support. Despite this fact, interventions including oral and topical chelation therapy are being used to treat autism following evaluation of hair, blood, or urine samples for heavy metal toxicity. In this study, hair samples were obtained from 15 children with autism between the ages of 2 and 6 years and 16 controls in the same age range who are the siblings of the subjects. The hair samples were obtained according to lab. specifications and submitted in a blind fashion to Doctor's Data Lab. for measurement of mercury levels. Data from the two groups were then compared using T-test. No significant differences were found between mercury levels for the two groups. This study raises questions about the theory that mercury toxicity causes autism and points to the difficulty in quantifying chronic mercury exposure through currently available laboratory measures.