OBJECTIVES: Symptoms of Restless Legs Syndrome (RLS) typically worsen in the early evening and nighttime affecting patients’ quality and duration of sleep which can have next day sequelae detrimental to daytime performance. The present qualitative research aimed to support the content validity of a new self-reported outcome measure to document the impact of disturbed sleep due to RLS on next day functioning. METHODS: An initial conceptual framework was developed based upon a review of the literature.  The development of the Restless Legs Syndrome – Next Day Impact (RLS-NDI) questionnaire included: concept elicitation interviews with 20 clinician-confirmed idiopathic RLS patients in the US (aged 35-64); grounded theory data collection and analysis methods; achievement of saturation; and review by clinical and measurement experts to generate items, responses, and instructions for the new scale.  Cognitive interviews were conducted with an additional 15 idiopathic RLS patients (aged 25-68) to ensure understanding of the new measure, concept comprehensiveness, and to identify any necessary revisions to the items and conceptual framework.    RESULTS: Twenty-three items were generated from patient quotes obtained during the concept elicitation interviews.  Impacts on next day functioning spontaneously attributed to disturbed sleep due to RLS symptoms included: activities of daily living (i.e., work, household chores), cognitive functioning (i.e., concentration, forgetfulness, mental tiredness, alertness), emotional functioning (i.e., irritability, depressed mood), physical functioning (i.e., physical tiredness, active leisure activities), energy, daytime sleepiness, and social functioning (i.e., relationships, social activities/situations).  Concept saturation was achieved.  Modifications to questions and responses were based on feedback provided during cognitive interviews.  The final measure consists of 14 items assessed “today” and rated on a severity numeric rating scale.     CONCLUSIONS: The RLS-NDI is an evaluative tool with demonstrated content validity to assess the impact of disturbed sleep due to RLS symptoms on RLS patients’ next day functioning.