Background: Research on intellectual disability has been criticized for primarily addressing the situation of people in high-income countries. Objective: /Hypothesis. To determine whether MICS6 data on ‘functional difficulty associated with learning’ (FDAL) in low- and middle-income countries could be used as a proxy indicator for intellectual disability. Methods: Secondary analysis of nationally representative data collected in Round 6 of UNICEF's Multiple Indicator Cluster Surveys (MICS) on 244,915 children in 18 middle- and low-income countries. Results: The prevalence of FDAL in middle- and low-income countries was broadly similar to the estimated prevalence of intellectual disability in high-income countries. The association between risk of FDAL and household wealth was weak, with alternative measures of developmental delay showing significantly stronger associations with household wealth. The risk of making potential false negative errors in identifying FDAL increases as household wealth and level of maternal education decrease. The risk of making potential false positive errors in identifying FDAL is greater among more highly educated respondents, although this association is only statistically significant among older children. Conclusions: The use of FDAL as a proxy indicator for intellectual disability cannot be recommended given: (1) it would probably underestimate the overall prevalence of intellectual disability in middle and low income countries; and (2) it is likely to be overestimate prevalence among families with higher socio-economic position (SEP) and underestimate prevalence among families with lower SEP. © 2020 Elsevier Inc.