Oligoastrocytomas are glial tumours consisting of a mixture of neoplastic astrocytic and oligodendroglial cells. Genetic alterations of oligoastrocytomas include loss of heterozygosity of chromosomes 1p and/or 19q (LOH 1p/19q), typically occurring in oligodendrogliomas, and mutations of TP53, frequently occurring in astrocytomas. To investigate whether these neoplastic cell types in oligoastrocytomas have different genetic profiles, we examined the two different components of oligoastrocytomas in comparison with the histological diagnosis of the specific tumour area for LOH 1p/19q and TP53 mutations by using microdissection technique. We found a variety of lost markers for 1p and 19q, and the presence of two different TP53 mutations in the tumour samples. In the majority of cases (9/11), the oligodendroglial and astrocytic components of an individual oligoastrocytoma displayed the same genotype. We present two cases of biphasic oligoastrocytomas with aberrant findings, suggesting the coexistence of genetically and morphologically distinct tumour cell clones in these tumours. In one case, the oligodendroglial part of the tumour showed LOH19q, whereas the astrocytic part showed TP53 mutation (codon 273). In another case, we found LOH 1p/19q in the oligodendroglial component, but two retained areas on chromosome 1p in the astrocytic component of the tumour. No evidence was found for the coexistence of tumour cells with the two genotypical changes within the same morphological region of one individual tumour. The two cases of biphasic oligoastrocytomas in our sample that display a different genotype in the astrocytic and oligodendroglial part of the tumour show that different components of an oligoastrocytoma may be derived from different cell clones during neoplastic transformation.