Teleroentgenograms of the lower extremity in 244 children (123 boys and 121 girls) were made at six-month intervals from age seven until skeletal maturity. Lengths were measured to determine the growth remaining in the femur and tibia. The subjects were healthy, well-nourished, middle class Americans, mostly of Northwest European descent. The percentage of growth occurring at each lower extremity growth plate was determined by roentgenstereophotogrametric analysis. The growth occurring at the distal femoral and proximal tibial growth plates was determined by multiplying the proportion of growth occurring at each age by the longitudinal growth remaining. The proportion of growth occurring at each growth plate is not equal. Also, growth-plate activity is not constant or average throughout growth. Overall, approximately 70% of growth in the femur occurs at the distal growth plate. The proportion of growth occurring in the distal femoral growth plate in girls varies from 60% at seven years of age to 90% at age 14. Similarly, the contribution of the distal femoral growth plate in boys varies from 55% at seven years of age to 90% at age 16. Overall, the contribution of the proximal tibial growth plate is approximately 57%. In girls, this varies from 50% at seven years of age to 80% at age 14. In boys it varies from 50% at seven years of age to 80% at age 16. From the age of seven to skeletal maturity, the distal femur contributes approximately 1.3 cm per year to femoral growth, except in the last two years, when it contributes half that amount.(ABSTRACT TRUNCATED AT 250 WORDS)