This study aimed to explore the factors that contributed to patients’ failures to complete inpatient detoxification for drug and/or alcohol dependence. A review of case files at a 25-bedded detoxification unit in the United Kingdom was undertaken. During a 12-month period 503 patients accessed treatment. Treatment was predominantly offered to patients with alcohol-related problems (66%), patients with drug-related problems (22%), and patients with drug and alcohol-related problems (12%). The mean length of treatment was 2 weeks. Just more then one half of the sample had a previous criminal conviction, with 24% of all offenses being for violence. From the total admissions 35 (7%) breached the unit rules and 48 (10%) left treatment early against medical advice. Patients who breached unit rules were typically undertaking drug detoxification, had poorer preadmission preparation, and had poor engagement in the therapeutic programme. A high-risk time for breach was on the weekend, where the most commonly cited reason was illicit substance use. Patients who left treatment against medical advice typically had an admission for drug or drug/alcohol detoxification, lived closer to the unit, had previous criminal convictions, and had lower treatment engagement.