BACKGROUND: The properties of hypobaric spinal anesthesia are excellent in sensory and motor block with low concentration solutions, increased venous return, and hemodynamic stability. Much volume is needed with hypobaric spinal anesthesia because low concentration solutions are used. The purpose of this study was to examine the effects of the injected volume on the level of anesthesia in hypobaric spinal anesthesia during a total hip replacement. METHODS: Twenty-seven adult patients scheduled for a total hip replacement were assigned randomly to two groups, one with 0.1% (n = 14), and the other with 0.2% (n = 13) hypobaric tetracaine spinal anesthesia. 8 mg of tetracaine was used for male patients and 7 mg for female patients. Hence the injected volumes were 8 ml for male and 7 ml for female patients in 0.1% solution and 4 ml for male and 3.5 ml for female patients in 0.2% solution, respectively. Epinephrine was mixed to either solution in concentrations of 1:200,000. The speed of injection was fixed at the rate of 0.2 ml/sec. RESULTS: The height of sensory block with the 0.1% was one dermatome higher (T5.7) than with the 0.2% tetracaine solution (T6.7). The time for sensory block (11.4 min vs 12.3 min) and the time for motor block (10.0 min vs 15.0 min) with 0.1% was shorter than that with the 0.2% tetracaine solution. The duration of sensory block and the duration of motor block were not significantly different in either groups. Complete motor block was achieved in all patients. CONCLUSIONS: Both 0.1% and 0.2% hypobaric tetracaine spinal anesthesia are suitable for a total hip replacement. However we got better results with 0.1% than with 0.2% tetracaine such as rapid and high sensory block with rapid motor block.