BACKGROUND: Concentrations of certain substances in follicular fluid (FF) are related to fertilization outcome and early post-fertilization development. The study aim was to identify FF markers with which to predict embryo implantation potential. METHODS: Concentrations of selected hormones, cytokines and growth factors in individual FF samples obtained during assisted reproduction treatment were related with treatment outcomes. RESULTS: Mean concentrations of LH, growth hormone (GH), prolactin (PRL), 17β-estradiol (E2) and insulin-like growth factor (IGF)-I were higher, and that of interleukin-1 (IL-1) was lower, in treatment attempts leading to a clinical pregnancy as compared with those in which no pregnancy was established. Concentrations of FSH, progesterone, tumour necrosis factor-α and IGF-II were similar in successful and unsuccessful attempts. In successful attempts, LH and GH levels were higher in those follicles from which oocytes giving rise to transferred embryos (i.e. embryos with best morphology and fastest cleavage rate) originated, as compared with other follicles from which a mature oocyte was recovered but was cryopreserved for later use. CONCLUSIONS: FF levels of LH, GH, PRL, E2, IGF-I and IL-1 may serve to analyse cases of repeated assisted reproduction failures and to assess effects of modifications of the ovarian stimulation protocol.