Age, derived fat index (FI), and clinical score at the time of menarche were determined from records of 63 girls with cystic fibrosis (CF) and from 17 comparably aged but nonmenarchal girls with CF. The mean age of menarche in girls with CF was 14.5 years at a mean FI of 20.1% fat; thus, they were significantly older and had less fat than normal girls. Menarchal age was related to severity of illness. Amenorrheic girls with CF, mean age of 14 years, were significantly sicker and smaller. Gonadotropin and estradiol levels in 23 girls with CF were normal. The delay in menarche in CF appears to be closely related to severity of illness. Furthermore, although the relationship of puberty to body composition may or may not be coincidental, it is unlikely that puberty is initiated by a "critical body composition."