Sexual size dimorphism (SSD) is widespread throughout the animal kingdom. In Drosophila, sex differences in body size arise during larval development and result in females that are 20-40% larger than males. Signalling mechanisms that regulate larval growth and final body size have been identified, but how they are differentially regulated in males and females to produce SSD is not well understood. A recent study reported that the sex determination gene transformer (tra) acts in the fat body to promote female-specific body growth (Rideout et al. 2015, PLoS Genetics). Here we show that Sex-lethal (Sxl), an upstream gene in the sex determination pathway, functions in the nervous system to upregulate female body growth in a non-autonomous manner. Nervous system-specific knockdown of Sxl in females is sufficient to convert their body to a male size, completely abrogating SSD. Conversely, female body size in sxl mutants can be rescued by expressing Sxl in only the nervous system. We find that Sxl functions specifically in peptidergic and GABAergic neurons to promote body growth in females. Current research focuses on mapping the Sxl neural circuitry regulating female-specific growth.