During intermediate-late phases of human folliculogenesis, LH plays a key role in promoting steroidogenesis and growth of the leading follicle. Ovarian stimulation for assisted reproduction techniques usually consists of administering exogenous FSH in a low LH environment. Although an impairment in LH-dependent paracrine activities would be expected, multiple follicular growth is efficiently achieved in almost all patients. Thus, there appears to be a discrepancy between classical folliculogenesis models and data from IVF. This study examines the 'interface' between basic endocrinological and clinical evidence, in an attempt to answer two questions: is there an LH therapeutic window, and if there is, how can this be exploited in the practice of assisted reproduction? It also reviews the evidence that specific subgroups of women may benefit from LH supplementation during ovarian stimulation.