Background and objective: To assess how the Delfia time-resolved immunofluorometric assay can most conveniently and economically be used to differentiate normal prepuberty from complete idiopathic hypogonadotrophic hypogonadism (IHH, Kallmann’s syndrome). Subjects: 42 prepubertal boys aged 8.06-14.1 years and 11 adult male patients with IHH. Design and measurements: Blood samples were withdrawn at 20-min intervals for 8 h from 23.00 to 07.00. Samples from the 6 h commencing 1 h after sleep onset were analysed for LH by Delfia. Results: Mean LH over this 6-hour period discriminated between IHH and normal prepuberty after the age of 12.5 years (no IHH subject > 0.31 U/l, no prepubertal subject < 0.33 U/l). The maximum hourly mean LH value for each subject gave a greater degree of mutual exclusivity (no IHHsubject > 0.45 U/l, no prepubertal subject < 0.50 U/l). Conclusion: Kallmann’s syndrome patients can be distinguished from prepubertal boys aged 12.5 years or over by blood sampling every 20 min for 6 h, commencing 1 h after sleep onset. The pooling of these samples into six 1-hour samples and subsequent Delfia assay will yield six 1-hour mean LH concentrations for each subject. The highest of these six concentrations will give a value with mutual exclusivity between the two groups.