We investigated whether growth hormone (GH) treatment could accelerate the onset of puberty in patients with isolated GH deficiency (GHD). Of the 135 boys and 89 girls who started GH treatment before the onset of puberty and were followed up at Tanaka Growth Clinic, 83 boys and 51 girls who started GH treatment sufficiently earlier than the average age at onset of puberty of GHD patients (<10 years vs. 11.7 years for boys; <9.5 years vs. 11.4 years for girls) were analyzed. Age at onset of puberty significantly positively correlated to age at the start of GH treatment (boys: r = 0.427, p < 0.0001; girls: r = 0.302, p < 0.05). When the subjects were divided into two groups each: for boys, Groups A (n = 45) and B (n = 39), treatment was started at age <8 and 8 to <10 years, respectively; for girls, Groups A (n = 26) and B (n = 21), treatment was started at age <7 and 7 to <9.5 years, respectively, age at the onset of puberty was significantly lower in Groups A than in Groups B by the Mann-Whitney U test (boys: p < 0.01; girls: p < 0.05) and Kaplan-Meier log-rank test (boys: p < 0.01; girls: p < 0.05). These results indicate that GH treatment accelerates the delayed onset of puberty in patients with GHD. Heights at the onset of puberty in Groups A and B were not significantly different, suggesting that early treatment does not increase adult height.