One Hundred (100) outpatients with a diagnosis of bipolar I disorder and cocaine abuse/dependence, established by a structured clinical interview and confirmed by a psychiatrist, will participate. Eligible participants will be given a physical exam, including an eye exam with an ophthalmoscope to rule out serious medical illnesses and cataracts. Additionally, medical and psychiatric histories and baseline labs, including CBC and a liver panel will be obtained. Blood will be drawn for routine laboratory analyses including a complete blood count (CBC) and SMA-20 at baseline and exit. Women of childbearing potential will be given a urine pregnancy test. Baseline measures of psychiatric symptoms will be assessed with the Hamilton Rating Scale for Depression HRSD, Young Mania Rating Scale (YMRS), Inventory of Depression Symptomatology-Self-Report 30-item version (IDS-SR30), and the Psychobiology of Recovery in Depression III - Somatic Symptom Scale (PRD-III). Cocaine craving will be assessed with Cocaine Craving Questionnaire (CCQ). Drug use will be assessed by self-report, with the Addiction Severity Index (ASI) and a urine drug screen (UDS). Two UDSs are performed at baseline. Cocaine use in the past week (dollar amount spent/week and days used/week) will be assessed by patient self-report. Use of and craving for other substances (benzodiazepines, barbiturates, alcohol, opiates, phencyclidine, and cannabis) will also be assessed by self-report of dollar amount and days used in the past week, UDSs, and with 100-mm single item visual analog craving scales. Side effects will be assessed with the Abnormal Involuntary Movement Scale (AIMS), the Simpson-Angus Scale (SAS), and the Barnes Akathesia Rating Scale (BARS). Subjects will be randomized and be titrated up to 400 mg/day with additional flexible titration after that time to a maximum of 800 mg/day of Quetiapine or identical appearing placebo add-on therapy in a double- blind fashion for 12 weeks. At each weekly assessment subjects will be evaluated with the HRSD17, IDS-R30, YMRS, CCQ, and a UDS, and receive an upward titration of the study drug. The ASI will be repeated every four weeks. Further, participants will return one additional time each week for a UDS. At the end of the study an unblinded psychiatrist will provide standard open label treatment with Quetiapine until the participants can be transferred back to their usual treatment facility. Follow up and after care arrangements will be made for each participant near completion of the study.