D-Cycloserine to Enhance Cognitive Behavioral Therapy (CBT) for Acrophobia

Inclusion Criteria: 1. Males or females 18-65 years of age with a psychiatric diagnosis of acrophobia defined by DSM-IV criteria. 2. Willingness and ability to comply with the requirements of the study protocol. Exclusion Criteria: 1. A lifetime history of bipolar disorder, schizophrenia, psychosis, delusional disorders or obsessive-compulsive disorder; an eating disorder in the past 6 months; organic brain syndrome, mental retardation or other cognitive dysfunction that could interfere with capacity to engage in therapy; a history of substance (amphetamines, benzodiazepines, barbiturates, cocaine metabolites, marijuana, narcotics, and sedative hypnotics) abuse or dependence or alcohol abuse or dependence (other than nicotine) in the last 6 months or otherwise unable to commit to refraining from alcohol use during the acute period of study participation. 2. Patients with posttraumatic stress disorder and panic disorder within the past 6 months are excluded. Entry of patients with other mood or anxiety disorders will be permitted in order to increase accrual of a clinically relevant sample. Patients with significant suicidal ideation (MADRS item 10 score \> 3) or who have enacted suicidal behaviors within 6 months prior to intake will be excluded from study participation and referred for appropriate clinical intervention. 3. Patients must be off concurrent psychotropic medication (e.g., antidepressants, anxiolytics, beta blockers) for at least 2 weeks prior to initiation of randomized treatment. 4. Significant personality dysfunction likely to interfere with study participation. 5. Serious medical illness or instability for which hospitalization may be likely within the next year. 6. Patients with a current or past history of seizures. 7. Pregnant women, lactating women, and women of childbearing potential who are not using medically accepted forms of contraception (e.g., IUD, oral contraceptives, barrier devices, condoms and foam, or implanted progesterone rods stabilized for at least 3 months). 8. Any concurrent psychotherapy initiated within 3 months of baseline, or ongoing psychotherapy of any duration directed specifically toward treatment of acrophobia is excluded. Prohibited psychotherapy includes CBT therapy focusing on exploring specific, dynamic causes of the phobic symptomatology and provides management skills. General supportive therapy initiated \> 3 months prior is acceptable. 9. Prior non-response to adequately delivered exposure (i.e., as defined by the patient's report of receiving specific and regular exposure assignments as part of a previous treatment) will exclude participants from the study. 10. Patients with a history of head trauma causing loss of consciousness, seizure or ongoing cognitive impairment. 11. Patients receiving isoniazid. 12. Patients unable to understand study procedures and participate in the informed consent process.