We report a long term study on the effectiveness of chronic GnRH agonist treatment on final or near-final height in 26 patients (20 females and six males) with true precocious puberty (TPP). This study differs from other treatment studies in that the median age at onset of therapy was 4.7 yr for females and 6.2 yr for males, the youngest cohort of treated patients reported to date. We compared patients treated with GnRH agonists who attained final or near-final height with a historical control group of untreated children with TPP (n = 116) matched for mean age of pubertal onset, etiology of TPP (idiopathic or neurogenic), rate of progression, and sex ratio. The current mean height of GnRH agonist-treated females who began therapy at more than 5 yr of age (157.6 +/- 6.6 cm) is already significantly greater than the mean final height of untreated females (152.7 +/- 8.6 cm). The current mean predicted height of the treated females is 164.6 +/- 9.7 cm. The current mean height of females whose treatment was started before 5 yr of age is greater (164.1 +/- 7.7 cm) than that of females whose treatment began after 5 yr of age (157.6 +/- 6.6 cm). The final height of untreated children whose age of sexual precocity was less than 5 yr at diagnosis is significantly less than that of treated patients who were less than 5 yr when they developed TPP (P = 0.0006). The current mean height of GnRH agonist-treated males is 166.3 +/- 12.2 cm, and the current mean predicted height is 170.8 +/- 11.3 cm. This is in sharp contrast to the mean final height of untreated males (155.6 +/- 7.7 cm). The current predicted height correlates negatively with the age at initiation of treatment and the initial bone age and positively with height SD for bone age in the agonist-treated children. The current mean height deviation from target height is significantly less in the 20 treated females (-1 SD) than in 93 untreated females (-2.4 SD; P = 0.006). The mean final height deviation from target height in 23 untreated males (-3.7 SD) is significantly greater than the current height deviation from target height in 6 treated males (-1.7 SD; P = 0.03). The salutary effects of long term GnRH agonist therapy on stature are more clear-cut in the younger treated children. Young untreated children may have the worst outcome with respect to final height.(ABSTRACT TRUNCATED AT 400 WORDS)