Glutathione S-transferase (GST) theta is an enzyme to detoxify xenobiotic compounds. The gene GSTT1 has a null/present polymorphism in one of the targets for cancer susceptibility research. The polymorphism is classified into two types: present type with at least one present allele (heterozygote and homozygote) and null type without a present allele. Although one report showed a method to distinguish the heterozygote from the present homozygote, its use has been limited possibly owing to the difficulty of successful genotyping of long DNA sequences (1460 base pairs). This article reports an alternative method utilizing typical PCR primers specific to the null allele (566 base pairs) and the present allele (458 base pairs). All samples of the GSTT1 null genotype processed by the present PCR method (n = 331), were correctly classified as the null genotype by a conventional method, and the samples of heterozygous (n = 364) or present homozygous (n = 108) genotype as the present genotype; this indicates that it is appropriate for future research to utilize three genotypes of the GSTT1 null/present polymorphism.