Oligodendrogliomas may be divided into those with deletion of chromosomes 1p and 19q (Del+), and those without (Del-). Del+ tumours show better survival and chemoresponsiveness but the reason for this difference is unknown. We have investigated whether these subgroups differ in (a) apoptotic index, (b) the proportion of cells licensed for DNA replication but not in-cycle, and (c) the relative length of G1-phase. Fluorescence in situ hybridisation with probes to 1p and 19q was used to determine the deletion status of 54 oligodendrogliomas, including WHO grades II and III. The apoptotic index was determined using counts of apoptotic bodies. Replication-licensed non-proliferating cells were determined from the Mcm2 minus Ki67 labelling index, whilst the geminin to Ki67 ratio was used as a measure of the relative length of G1. Del+ oligodendrogliomas showed a higher apoptotic index than Del- tumours (P=0.037); this was not accounted for by differences in tumour grade or in proliferation. There were no differences in the Mcm2-Ki67 index or in the geminin/Ki67 ratio between the subgroups, but grade III tumours showed a higher proportion of licensed non-proliferating cells than grade II tumours (P=0.001). An increased susceptibility to apoptosis in oligodendrogliomas with 1p+/-19q deletion may be important in their improved clinical outcome compared to Del- tumours.