The precision of the topographic projection from the isthmo-optic nucleus (ION) to the retina has been examined in chicken embryos and chicks by the retrograde transport of a fluorescent carbocyanine dye from restricted retinal sites. At all ages, the labeled neurons are most numerous in the topographically appropriate part of the ION, but in younger embryos up to 49% of them are found outside this region. The distribution of these "aberrantly" projecting neurons is variable, but they generally occur throughout the entire ION. They all die during the ION's period of neuronal death, accounting for most of the 60% cell loss that then occurs. We therefore suggest that a major role of neuronal death during brain development is to reduce the imprecision of neuronal connections.