The criterion tumor volume (TV) for clinically insignificant prostate cancer has been reported, but it differs from study to study: some have reported TV < 200 mm(3); others, < 500 mm(3). The aim of the present study was to distinguish clinically insignificant cancers from significant ones using molecular biological methods. A total of 184 microscopic cancers (MC) defined as limited within a 3 mm circle and 82 main tumor (MT) nodules were selected. Thirteen microsatellite loci at 6q22, 8p23.2-23, 13q14 and 13q33 were evaluated for loss of heterozygosity (LOH). MT were subgrouped as TV > or = 500 mm(3) or < 500 mm(3); TV > or = 200 mm(3) or < 200 mm(3); and TV < 200 mm(3), 200 mm(3) < or = TV < 500 mm(3) or TV > or = 500 mm(3); and frequencies of LOH were compared between these three groups. Frequencies of LOH at 6q16-21, 6q22, 8p23.1, 8p23.2, 13q14 were significantly lower in MC (1.0%, 2.7%, 1.9%, 1.1% and 5.4%) than in MT (30.9%, 40.4%, 12%, 8.7% and 20.6%), but no significant differences in LOH frequency were found within each of the three TV groups, between each cut-off. When insignificant tumor is defined as TV < 200 mm(3) or < 500 mm(3), it should include tumors with malignant potential equivalent to larger tumors. It is suggested that in order to identify insignificant tumor within a strict safety range, TV should be set lower.