Secretion of leptin from adipocytes communicates body energy status to the brain by activating the leptin receptor long form (LRb). LRb regulates energy homeostasis and neuroendocrine function; the absence of LRb in db/db mice results in obesity, impaired growth, infertility and diabetes^,,,. Tyr 1138 of LRb mediates activation of the transcription factor STAT3 during leptin action^,,,. To investigate the contribution of STAT3 signalling to leptin action in vivo, we replaced the gene encoding the leptin receptor (lepr) in mice with an allele coding for a replacement of Tyr 1138 in LRb with a serine residue (lepr^S1138) that specifically disrupts the LRb–STAT3 signal. Here we show that, like db/db mice, lepr^S1138 homozygotes (s/s) are hyperphagic and obese. However, whereas db/db mice are infertile, short and diabetic, s/s mice are fertile, long and less hyperglycaemic. Furthermore, hypothalamic expression of neuropeptide Y (NPY) is elevated in db/db mice but not s/s mice, whereas the hypothalamic melanocortin system is suppressed in both db/db and s/s mice. LRb–STAT3 signalling thus mediates the effects of leptin on melanocortin production and body energy homeostasis, whereas distinct LRb signals regulate NPY and the control of fertility, growth and glucose homeostasis.