Background/Aims: Limited data are available about spontaneous growth, pubertal growth spurt and the long-term outcome of patients suffering from familial male precocious puberty (FMPP). We report on a boy with FMPP whose growth pattern and pubertal development was studied longitudinally without treatment. Methods: Long-term prospective follow-up without treatment of a 6.2-year-old boy with FMPP having inherited a mutation of the LH receptor gene (A568V) from his father. Results: The pubertal growth spurt was of unusual maximal amplitude (growth rate 12.4 cm/year at the age of 5–6 years) and of extraordinary duration lasting for 5.2 years from age 3.8 to 9.0 years. No deterioration of height potential was observed. Height (174 cm) was within target height range (171.5–188.5 cm) at age 13 years. No central precocious puberty occurred. Conclusion: FMPP is an experiment of nature demonstrating that the amplitude and duration of the pubertal growth spurt are much more variable than previously described. Furthermore, this case emphasizes that the indication for treatment is highly dependent on intrafamilial and individual factors.