PURPOSE: To develop and validate regression models to estimate maximal oxygen uptake (VO₂max) from the new estimation formula based on Astrand-Ryhming step test protocol (AR protocol) in Korean male and female youths aged 13-18 years. METHODS: One hundred twenty participants performed maximal graded exercise test (GXT) on a treadmill and Astrand-Ryhming step test (AR test) to measure VO₂max and VO₂max estimates, respectively. Participants in the validation study performed the GXT with the stationary metabolic cart and the Astrand-Ryhming step test twice for test-retest reliability. All tests were conducted at least three days apart and in a random order. RESULTS: Mean calculated VO₂max (40.43±11.57 mL/kg/min) from the Astrand-Ryhming step test equation was significantly low from that measured during the GXT from stationary metabolic cart (47.32±8.75 mL/kg/min, p<.01). New regression equations were developed from present data to predict VO₂max for male and female youths: y=(-13.963×sex)-(0.216×W)-(0.072×HR)+91.296 (r=.772, SEE=5.64 mL/kg/min). The VO₂max (47.22±6.76 mL/kg/min) of modified Astrand-Ryhming step test equation showed no significant difference from the VO2max measured through GXT. CONCLUSIONS: It is concluded that this equation developed in this study provides valid estimates of VO₂max of Korean male and female youths aged 13-18 years. A further study needs to develop a step test estimation equation targeting adults and elderly people in which validity and reliability are secured.