The use of a new, more sensitive immunoassay for growth hormone (GH) revealed that the serum levels in men were lower than expected in sera drawn ambulatory in the morning after an overnight fast and that the gender difference was more than 10 times greater than reported. These observations led to a more thorough study on the impact of gender and sex steroids on the levels of GH and other hormones in ambulatory morning samples and over a 24-hour period. Furthermore, the impact of gender was studied in GH deficient (GHD) patients and healthy young adults treated with GH, and in patients with acromegaly treated with octreotide. An 80-fold gender difference in the morning GH levels was observed in young individuals as a reaction to ambulation, with decreased levels in men and increased in women. Oral contraceptives (OCs) given to women further increased the morning GH levels. During the day, higher outputs of epinephrine and lower levels of GH were seen in the men, while no gender differences were seen at night. The gender difference in morning GH levels decreased with age due to opposite changes in men and women. Administration of 17β-estradiol (E2) via subcutaneous implants in postmenopausal women, which increased the E2-concentrations to luteal phase levels, had no effect on the morning GH levels, indicating that the different reactions to ambulation do not appear to result from a direct sex steroid effect alone. Short-term administration of GH to young, healthy adults resulted in larger effects on insulin-like growth factor I (IGF-I) and other key metabolic parameters in men than in women. The smallest response was noted in women taking OCs. The clinical studies involving long-term GH treatment of patients with GHD demonstrate a gender difference in GH responsiveness, with women being less sensitive than men, a fact which should have a therapeutic impact in patients with GH disorders. A further gender difference of therapeutic importance was observed in men and women with acromegaly. Long-term treatment with a slow-release formulation of octreotide resulted in higher IGF-I levels in the men, despite equal doses of the drug and similar levels of GH.