Background: Hyperinsulinism and hyperandrogenism have the capacity to increase bone mineral density (BMD) and serum leptin, independently of body fat mass. We therefore assessed lumbar BMD and serum leptin in girls with the sequence of a low birthweight and precocious pubarche (PP) in childhood, in whom hyperinsulinism and hyperandrogenism have been described. Methods: Fifty-two non-obese PP girls were studied (age range 6.9–14.9 years). Serum leptin was also measured in 42 control girls, matched for age, body mass index and pubertal stage. Results: BMD SDS, measured by dual-energy X-ray absorptiometry, was elevated in PP girls compared to the population reference (0.39 ± 0.18 SDS; p = 0.03) and bone age, assessed from hand radiographs, was significantly advanced compared to chronological age (1.2 ± 0.1 years; p < 0.0005). Conclusion: Compared to control girls, PP girls had higher leptin levels for degree of body mass index (PP girls: 9.4 ± 0.6 ng/ml; controls: 7.8 ± 0.6 ng/ml; p = 0.01). In PP girls, serum leptin was inversely related to birthweight (r = –0.32, p = 0.01) and positively related to free androgen index (FAI) (r = 0.71, p < 0.0005). BMD SDS was also inversely related to birthweight (r = –0.26, p < 0.05) and positively related to serum leptin (r = 0.42, p < 0.05), FAI (r = 0.45, p < 0.05) and mean serum insulin during oral glucose tolerance testing (MSI) (r = 0.59, p < 0.0005). In multiple regression, MSI was the strongest determinant of BMD SDS (β = 0.50, p = 0.002). In conclusion, elevated BMD and serum leptin in non-obese PP girls were related to degrees of low birthweight, hyperinsulinism and hyperandrogenism. The characteristic hyperinsulinism of PP girls is proposed to be the key variable in this constellation.