ABSTRACT: BACKGROUND: For critically ill patients treated in intensive care units (ICU), two feeding strategies are currently being advocated, one by American/Canadian and the other by European expert guidelines. These guidelines differ particularly in the timing of initiating parenteral nutrition (PN) in patients for whom enteral nutrition (EN) does not reach caloric targets. METHODS: The EPaNIC trial is an investigator-initiated, non-commercial, multi-center, randomized, controlled, clinical trial with a parallel group design. This study compares early (European guideline) versus late (American/Canadian guideline) initiation of PN when EN fails to reach a caloric target. In the early PN group, PN is initiated within 24-48 hours after ICU admission to complete early enteral nutrition (EN) up to a calculated nutritional target. In the late PN group, PN completing EN is initiated when the target is not reached on day 8. In both groups, the same early EN protocol is applied. The study is designed to compare clinical outcome (morbidity and mortality) in the 2 study arms as well as to address several mechanistical questions. We here describe the EPaNIC study protocol and the statistical analysis plan for the primary report of the clinical results. DISCUSSION: The study has been initiated as planned on august 01 2007. One interim analysis advised continuation of the trial. The study will be completed in February 2011. Trial registration NCT 00512122 http://clinicaltrials.gov/ct2/show/NCT00512122.