This article describes novel methods of applying the Ages and Stages Questionnaire--3rd edition (ASQ-3) to assess and quantify developmental delay among children following the 2015-2016 Zika virus outbreak in Brazil. Many of the children with Zika virus infection were expected to have severe developmental delay. However, administering the ASQ-3 to caregivers of these children according to standard protocol would have screened for the overall presence of delay but not the severity of delay. We adopted an amended protocol for administration of the ASQ-3 to quantify the developmental functioning of children severely affected by Zika virus infection in this investigation. Protocols for administering the ASQ-3 among this population were drafted in consultation with developmental measurement experts and are presented here. Specific developmental estimates are discussed, including developmental age equivalents, developmental quotients, and developmental quotient z scores. The calculations of these estimates are presented with examples in the context of the 2015-2016 Zika virus outbreak and associated microcephaly among prenatally infected children from 2 states in northeastern Brazil. Potential applications of these methods for estimating developmental ability among similar pediatric populations are discussed.