To determine the earliest signs of pubertal maturation, we followed 515 boys, aged 10 to 15 years at intake, every 6 months for 3 years. Changes in age, height, weight, body mass index, and serum levels of sex steroid hormones were significantly related to pubertal stage (PS). The earliest clinical stage of pubertal maturation, designated PS2a, was represented by the absence of public hair and a testicular volume 3 cc or greater; 6 months later, further maturation had occurred in 82% of these boys. Inclusion of PS2a as the earliest stage of puberty may help allay concerns about boys with perceived delayed maturation, and may allow more precise definition of early puberty.