In order to examine the effectiveness and safety of hydraulic resistance strength training in young males, 26 pre-pubertal males (mean age = 8.2 +/- 1.3 yr) completed a 14-wk strength training study. Subjects were evaluated before and after the 14-wk experimental period for pubertal state (Tanner's sexual maturity rating, serum testosterone, and serum dihydroepiandrosterone sulfate). Effectiveness of the strength training program was determined by measuring pre-post differences in: isokinetic strength for flexion and extension at the knee and elbow joints at two speeds (30 degrees and 90 degrees X s-1) (KIN COM, Chattecx, Inc., Chattanooga, TN), flexibility, standing long jump, vertical jump, body composition parameters, maximal oxygen consumption, and creatinine phosphokinase. Safety of strength training was assessed by biphasic musculoskeletal scintigraphy before and after the program and by physician evaluation of complaints by subjects. Strength training subjects (N = 16) participated in a 45 min/session, 3 session/wk, 14-wk supervised strength training program with an attendance rate of 91.5%. Participants performed concentric work using hydraulic resistance equipment (Hydra-Fitness Industries, Belton, TX). Eccentric work was not performed. Control subjects (N = 10) did not strength train but did participate in sport activities and activities of daily living. Results indicated that strength training subjects increased isokinetic strength as a result of strength training (average concentric work/repetition increases by 18.5 to 36.6% for the eight motions tested; torque scores over the first 90% of the range of motion increases by 13.1 to 45.1% for the eight motions tested). These changes were significantly greater than changes seen in the control group (P less than 0.05). Strength training subjects also demonstrated significant improvements (as compared to control subjects) in vertical jump (+10.4%), flexibility (+8.4%), and maximal oxygen consumption [+19.4% (l X min-1), +13.8% (ml X kg X min-1)] after the experimental period. Musculoskeletal scintigraphy revealed no evidence of damage to epiphyses, bone, or muscle as a result of strength training. Only one strength training-related injury was reported (left shoulder pain, 3 strength training sessions missed). In contrast, six strength training subjects sustained injuries during activities of daily living, resulting in 47 missed strength training sessions. It was concluded that, in the short term, supervised concentric strength training using hydraulic resistance equipment is safe and effective in pre-pubertal boys.