Pubertal development between 8 and 18 years is reported in 142 Swiss males of the First Zurich Longitudinal Study (1954-1980). Genital and pubic hair development were rated according to TANNER [14]. Testicular growth was assessed by comparative palpation with the orchiometer of PRADER [10]. The age at which pubertal development was initiated varied between the eighth and the fifteenth year of life. Reaching a testicular volume of 3 ml was found to be the most reliable indicator for the onset of pubertal development. The various pubertal stages of genital and pubic hair development tended to occur at earlier ages in this study than previously reported. This was particularly true for PH6 which was observed in 73% of the boys by age 20. Standard deviations of 1.0 to 1.5 years for the ages at which the pubertal stages were reached reflected the large variations in the timing of pubertal development. The mean duration of genital development was 3.5 +/- 1.1 years, of pubertal penis growth 1.8 +/- 0.7 years, of pubertal testicular growth 3.5 +/- 1.0 years and of pubic hair development 2.7 +/- 1.0 years. Moderate correlations were found between the ages at which genital development, pubic hair development and pubertal testicular growth were initiated. The ages at completion of these pubertal characters and the age at which the peak of the adolescent growth spurt was reached were highly positively correlated with each other. For a given stage of a pubertal character at least 80% of the children were within two successive stages of any other pubertal character, e.g. 87% of the boys were either in G2 or G3 when they reached PH2. No significant relationship between the durations of genital development, pubic hair development and pubertal testicular growth could be established. Likewise no significant relationship could be found between the timing and duration of pubertal development.