AZT has been effective in treating some patients with AIDS, and SMX/TMP is an antibiotic combination which is useful in preventing or treating Pneumocystis carinii pneumonia (PCP), which is an important cause of disease and death in patients with AIDS. It is important to know how drugs interact in patients because addition of a second drug may change the speed at which a drug is eliminated from the body, and cause increased toxic effects or decreased therapeutic effects.