Serum gonadotropin and sex steroid levels were measured in 106 patients with cystic fibrosis (CF), 46 males and 60 females, aged 8 to 24 years. The finding of delayed pubertal increments of serum gonadotropin and sex steroid levels in CF patients suggests late maturation of the reproductive endocrine system. Although pubertal changes in reproductive endocrine hormones in patients with CF appear to be temporally delayed, generally appropriate levels of these hormones are finally attained in most patients by the late teenage years. Delayed maturation of the reproductive endocrine system probably is secondary to hypothalamic-pituitary dysfunction, the result of chronic inanition.