Prolactin deficiency can occur alone, as can be seen by the deficiency of 1 or more of the other anterior pituitary hormones. Isolated prolactin deficiency is an extremely rare condition, and the only known symptom is the lack of lactation in postpartum women. A 29-year-old female presented in 2 weeks postpartum for evaluation of lactation failure. On the postpartum 16th day, the prolactin level was very low. Since there were no other pituitary abnormalities, we considered the diagnosis of isolated prolactin deficiency. In this case, we presented lactation failure due to isolated prolactin deficiency, which is extremely rare in the literature. In isolated prolactin deficiency, most of the patients are diagnosed with the absence of lactation and there are no other concurrent pituitary pathologies. In the treatment, recombinant human prolactin stands out as an option, but it is unavailable in many countries. In most cases, baby formula is used to feed babies.