ObjectiveNo instrument has been developed and validated across cultures to measure the degree of support provided by informal carers to people with schizophrenia. We aimed to develop such a measure. MethodThe Maristan Scale of Informal Care was developed directly from the views of patients with schizophrenia in six countries. Face-to-face interviews were carried out with participants and 103 were repeated after 30days. Principal Axis Factoring followed by Promax rotation evaluated the structure of the scale. Horn's parallel combined with bootstrapping determined the number of factors. Cronbach's alpha estimated the scale's internal consistency and intra-class correlation its test-retest reliability. ResultsA total of 164 interviews were undertaken, 103 with re-test. The Horn's Parallel Analysis and the analysis of the Promax rotation revealed one factor. Cronbach's alpha was 0.89. Intra-class correlation coefficient was 0.56 (95\% CI 0.42-0.68) and this increased to 0.64 (95\% CI 0.51-0.75) after removing two outlying values. Patients from Argentina recorded the lowest scores (poor informal support/care). ConclusionThe Maristan Scale of Informal Care is a reliable instrument to assess the degree of support provided by informal carers to people with schizophrenia across cultures. A confirmatory factor analysis is needed to evaluate the stability of its factor structure.