This study aimed to optimize a method to estimate liver volume by SPECT. Several phantoms were used to simulate the liver shape with a volume between 400 and 2,500 cc. The SPECT studies were acquired using 360 degrees and 180 degrees circular orbits, 64 projections, 64 x 64 word matrix, all purpose parallel-hole collimator and were reconstructed with a HANN filter with frequency cutoff = 0.5 cycles/ pixel. The reconstructed studies were processed with a program that estimates the organ volume for different threshold cutoff levels for counting. The optimum cutoff threshold level to quantify the liver volume was 50% and 48% for 360 degrees orbits and 180 degrees orbits, respectively. A linear relationship was observed between the real and calculated volumes with r = 0.994 for the 360% orbits and r = 0.976 for the 180 degrees orbits, The average error between corrected and real volumes was 4.2% and 10.87% for both orbits, respectively. The method was applied to 14 patients with different degrees of hepatomegaly who had undergone a liver 99mTc-sulfur colloid scintigraphy. The intra and interobserver variability as well as the influence of the degree of contour adjustment of the phantom mask were studied. No significant differences were observed in the results, so that it can be considered that neither the mask trace nor the operator influence the results.