Human immunodeficiency virus (HIV) infection interferes with a person's immune system. Fortunately, treatment with combinations of several anti-HIV drugs greatly improves health outcomes for people with HIV infection. Unfortunately, patients have difficulty taking the drugs correctly. Treatment success depends heavily on patients' taking the anti-HIV drugs exactly as prescribed and absorbing appropriate amounts of the drugs. Low levels of the drugs in the patient's body can be due to missed doses or to problems with absorption of the drugs into the body. Patients who miss doses of anti-HIV drugs or who do not absorb the drugs adequately can develop resistant forms of the virus. “Resistant” means that the drugs no longer effectively fight the virus. Measuring the levels of drugs in the blood and asking patients about missed drug doses are not accurate ways to tell whether patients took and absorbed the anti-HIV drugs over previous months. Doctors need better ways of testing to see whether patients with HIV infection have appropriate levels of the drugs.