Objective: To validate a measure of the carer's perspective of a stroke survivor's communication in everyday life. Design: Cross-sectional, interview-based, psychometric study. Setting: A community sample from the northwest of England, UK. Subjects: Fifty-eight carers and 58 stroke survivors with communication problems (aphasia and/or dysarthria) following a stroke within the previous 4-12 months. Interventions: Administration of the 20-item Carer Communication Outcome after Stroke (Carer COAST) scale, on two occasions, within a two-week period; the 15-item Carers of Older People in Europe (COPE) Index, the patient Communication Outcome after Stroke (COAST) Scale, and collection of demographic and other data relating to the stroke survivor's disability (Barthel Index), degree of aphasia (Frenchay Aphasia Screening Test) and hospital diagnosis of aphasia/dysarthria. Main measures: Acceptability (missing values), reliability (internal consistency and test-retest reliability) and construct validity. Results: Carer COAST showed good acceptability (no incomplete items, sample spread 24-100%), internal consistency and test-retest reliability for the scale (α = 0.94; intraclass correlation (ICC) = 0.91) and its subscales (α = 0.78-0.90; ICC = 0.75-0.87), and indicative evidence on construct validity (Carer COAST, COPE subscales and COAST). There were statistically significant correlations between the communication items of Carer COAST and the negative impact of caregiving (rs = -0.29) and the financial difficulties of caregiving (rs = -0.38). Conclusions: The Carer COAST scale has considerable potential as a reliable and valid measure of the carer's perspective on the communication effectiveness of stroke survivors. Intercorrelations with COPE provide specific evidence of the impact of caring for a person with communication difficulties following a stroke. © SAGE Publications 2009.