The proliferative response of non-β islet endocrine cells in response to type 1 diabetes (T1D) remains undefined. We quantified islet endocrine cell proliferation in a large collection of nondiabetic control and T1D human pancreata across a wide range of ages. Surprisingly, islet endocrine cells with abundant proliferation were present in many adolescent and young-adult T1D pancreata. But the proliferative islet endocrine cells were also present in similar abundance within control samples. We queried the proliferating islet cells with antisera against various islet hormones. Although pancreatic polypeptide, somatostatin, and ghrelin cells did not exhibit frequent proliferation, glucagon-expressing α-cells were highly proliferative in many adolescent and young-adult samples. Notably, α-cells only comprised a fraction (∼1/3) of the proliferative islet cells within those samples; most proliferative cells did not express islet hormones. The proliferative hormone-negative cells uniformly contained immunoreactivity for ARX (indicating α-cell fate) and cytoplasmic Sox9 (Sox9^Cyt). These hormone-negative cells represented the majority of islet endocrine Ki67+ nuclei and were conserved from infancy through young adulthood. Our studies reveal a novel population of highly proliferative ARX+ Sox9^Cyt hormone-negative cells and suggest the possibility of previously unrecognized islet development and/or lineage plasticity within adolescent and adult human pancreata.