Background. There is a wealth of evidence regarding the detrimental impact of excessive alcohol consumption on the physical, psychological and social health of the population. There also exists a substantial evidence base for the efficacy of brief interventions aimed at reducing alcohol consumption across a range of healthcare settings. Primary research conducted in emergency departments has reinforced the current evidence regarding the potential effectiveness and cost-effectiveness. Within this body of evidence there is marked variation in the intensity of brief intervention delivered, from very minimal interventions to more intensive behavioural or lifestyle counselling approaches. Further the majority of primary research has been conducted in single centre and there is little evidence of the wider issues of generalisability and implementation of brief interventions across emergency departments. Methods/design. The study design is a prospective pragmatic factorial cluster randomised controlled trial. Individual Emergency Departments (ED) (n = 9) are randomised with equal probability to a combination of screening tool (M-SASQ vs FAST vs SIPS-PAT) and an intervention (Minimal intervention vs Brief advice vs Brief lifestyle counselling). The primary hypothesis is that brief lifestyle counselling delivered by an Alcohol Health Worker (AHW) is more effective than Brief Advice or a minimal intervention delivered by ED staff. Secondary hypotheses address whether short screening instruments are more acceptable and as efficient as longer screening instruments and the cost-effectiveness of screening and brief interventions in ED. Individual participants will be followed up at 6 and 12 months after consent. The primary outcome measure is performance using a gold-standard screening test (AUDIT). Secondary outcomes include; quantity and frequency of alcohol consumed, alcohol-related problems, motivation to change, health related quality of life and service utilisation. Discussion. This paper presents a protocol for a large multi-centre pragmatic factorial cluster randomised trial to evaluate the effectiveness and cost-effectiveness of screening and brief interventions for hazardous alcohol users attending emergency departments. Trial Registration. ISRCTN 93681536.