STUDY QUESTION: What is the course of the LH/FSH ratio from infancy into adulthood in healthy individuals and in patients with Differences of Sex Development (DSD)? SUMMARY ANSWER: The LH/FSH ratio had a marked overlap between the sexes after infancy and onwards throughout adulthood in healthy individuals and it was not a marker of hypogonadism in DSD patients. WHAT IS KNOWN ALREADY: The LH/FSH ratio is a distinct marker of sex during minipuberty. No study has evaluated the LH/FSH ratio from infancy into adulthood. STUDY DESIGN, SIZE, DURATION: This was a combined study of prospective longitudinal and cross-sectional cohorts of healthy individuals totaling 6417 males and females aged 0-80 years. Retrospective data from a single, tertiary center on 125 patients with DSD was also included. PARTICIPANTS/MATERIALS, SETTING, METHODS: Based on the healthy males (n = 3144) and females (n = 3273) aged 0-80 years, reference ranges for LH, FSH and the LH/FSH ratio were established from infancy (after minipuberty) and onwards. LH, FSH, and the LH/FSH ratio in 125 patients with DSD not undergoing treatment were compared to the reference ranges. Included DSD diagnoses were: Klinefelter syndrome including mosaic variants (males: n = 14), Turner syndrome including mosaic variants without Y-chromosome material (females: n = 48), 45,X/46,XY mosaicism (males: n = 24 and females: n = 6), partial androgen insensitivity syndrome (males: n = 11), complete androgen insensitivity syndrome (females: n = 13) and anorchia (males: n = 9). MAIN RESULTS AND THE ROLE OF CHANCE: An overlap was observed in the LH/FSH ratio reference curves between males and females. However, when comparing the sexes at specific time points, the LH/FSH ratio was significantly higher in healthy males during childhood and adulthood and significantly higher in healthy females during puberty. When compared with healthy participants, male patients with anorchia and 45,X/46,XY mosaicism had significantly lower ratios, while patients with androgen insensitivity, regardless of sex, had significantly higher ratios. LIMITATIONS, REASONS FOR CAUTION: The limitations of this study include that; (i) all healthy individuals were Caucasian, so conclusions may not apply to non-Caucasians; (ii) the calculated LH/FSH ratios were restricted to the specific analytical method used and may not be applicable to other laboratories; (iii) the samples from healthy individuals were stored for varying amounts of time up to 20 years which may affect the durability; and (iv) DSD diagnoses are heterogeneous thus making sturdy conclusions across diagnoses impossible. WIDER IMPLICATIONS OF THE FINDINGS: In this study of combined cohorts of healthy participants, the largest normative ranges of LH, FSH, and the LH/FSH ratio to date were created. These reference ranges provide the opportunity for clinical as well as research use for all three markers. However, the previously rather undescribed LH/FSH ratio was not a distinct marker of sex after infancy nor a new marker of hypogonadism. Although there were significant differences between subgroups of DSD patients compared to healthy controls, the clinical significance of the LH/FSH ratio after infancy lacked. However, it can be speculated whether there are other areas of clinical application not investigated in this article, for example as a marker of fertility in select patient groups. As gonadotropin assays are readily available and gonadotropin measurements are part of regular workups, the LH/FSH ratio can easily be explored in further research without additional costs. STUDY FUNDING/COMPETING INTEREST(S): M.L.L. was funded by the Absalon Foundation. Cohort 1 was funded by the European Commission, through the Biomed 2 Program (BMH4-CT96-0314), Environmental Reproductive Health (QLK4-CT1999-01422) and EXPORED (QLK4-2001-00269), by the Danish Council for Independent Research (9700833 and 9700909), and by the Svend Andersens Foundation. Cohort 2 was funded by the Danish Environmental Research Program (96.01.015.16.05). Cohort 3 was funded by Kirsten and Freddy Johansens Foundation.NA. DATE OF FIRST PATIENT’S ENROLMENT: June 1990 (the launch of the department from which this project stems).