Conditioned opioid phenomena have now been documented in both opiate-dependent and abstinent patients. Although the existence of these phenomena is generally accepted, their potential role in drug use and relapse is still under study. Our own research indicates that both physiologic and subjective responses may be conditioned, and that these responses are present in a significant proportion of patients who abuse opiates. Patients' responsivity (for example, craving or a reduction in skin temperature) may vary depending upon mood, cognitive set, and the individual relevance of the drug-related stimuli. Extinction procedures have been developed that are effective in reducing both subjective and physiologic conditioned responses. The clinical impact of these extinction procedures is now being assessed in a large-scale treatment-outcome study with abstinent patients. The results of this project will help determine the actual clinical significance of these complex but potentially important phenomena.