Evaluation of mercury exposure in an individual patient ideally includes the presenting history, physical examination, consideration of the differential diagnosis, and mercury analysis of blood and urine specimens. Analysis of mercury in hair specimens may supply useful supplemental information about exposure to organic compounds such as methylmercury, particularly to help reconstruct the pattern of prior exposure. The most appropriate specimen is generally terminal-type hair from the occipital-neck junction, clamped to maintain strand alignment, and oriented to the scalp. Hair from the initial 0.5 cm adjacent to the scalp represents on average 1-3 wk before collection, and consideration of the time frame represented by the specimen is an important part of the evaluation. Literature reports describe hair mercury levels as high as 2400 μg/g. Hair mercury level is usually <1 μg/g in individuals who do not eat fish but may be >30 μg/g in those who frequently consume fish with high mercury content. Hair mercury level is often not correlated with blood mercury concentration or symptoms of mercury toxicity, and reports of hair contamination by exogenous mercury are not uncommon. Hair mercury level is notoriously prone to misinterpretation and should be used with an understanding of its limitations.