(FORTUNE Magazine) – Beethoven didn't take morphine, but poet John Keats used opiates. We know because new tests using people's hair--even the hair of people long dead--reveal a great deal about their health and personal habits.

Not surprisingly, in an age when 81% of large U.S. corporations engage in some kind of employee drug screening, a lot of companies are looking hard at these new testing techniques. According to the American Management Association, about 2% of big U.S. companies have already begun to test hair, and the percentage is rising. A major selling point is that hair analysis has a so-called wide detection window. While urine tests generally detect drug use during the previous week, hair tests can reveal any illegal indulgence back as far as a month or more.

Also not surprisingly, employees at some companies that have abandoned plastic cups for barber's shears are balking at the new testing policies. Workers at Bic Corp., the retail chain Sports Authority, and General Motors have complained. Their argument is that hair analysis can expose genetic information contained in DNA, such as hereditary defects or a predisposition to certain diseases. Theoretically, insurance companies could use such data to deny coverage, or a company could bar promotions because someone is likely to suffer a major illness in later life.

The courts will eventually decide the issue. Two former employees of Vyvx, an Oklahoma-based fiber-optics firm, lost their jobs because they wouldn't submit to hair testing. They filed a wrongful termination suit, charging that the practice should be banned because the results could be used for genetic screening. Not likely, replies CEO Ray Kubacki of Psychemedics Inc., the firm that handled the testing for Vyvx. Says he: "Psychemedics only does testing for drugs of abuse. We do not do genetic testing. Even if a company asked us, we would not do it."

--Sheree R. Curry