Human immunodeficiency virus (HIV, the virus that causes AIDS) interferes with the ability to fight infections. Simultaneous use of several anti-HIV drugs can reduce the amount of the virus in the blood and improve patient outcomes. Unfortunately, HIV is not always fully suppressed by such treatments. HIV can then become resistant to the drugs, and the patient must then change to another group of drugs. Two blood tests, the number of CD4 immune cells (a higher number is better) and viral load (a smaller load is better), usually guide doctors in making changes in anti-HIV drug regimens for their patients. To use these tests most effectively to make treatment decisions, doctors need a method to predict which patients are most likely to become resistant to the drugs. Then, physicians could monitor such patients especially closely for early signs of treatment failure and quickly change treatments.