There is currently no means of identifying the subgroup of APL patients who will succumb to haemorrhagic or thrombotic complications. We have investigated factor V Leiden and thermolabile methylene tetrahydrofolate reductase (MTHFR) to determine whether these are of predictive value for thrombosis in the context of APL. Of 48 patients drawn from the MRC ATRA trial, two were heterozygous for factor V Leiden (allele frequency 2.1%). 10 homozygotes and 17 heterozygotes for thermolabile MTHFR were identified (allele frequency 38.5%). Amongst these patients, one thrombosis occurred (thermolabile MTHFR heterozygote). In the group with no identified increased thrombotic risk, three episodes were recorded. This approach failed to predict thrombotic events in APL, although the exact implications of specific genotypes remain to be established by larger studies.