We describe a simple PCR method to detect highly polymorphic repetitive DNA markers (microsatellites) in leukaemia using DNA from fresh and archival bone marrow smears. Neither the period of storage nor staining of the slides affected microsatellite PCR. Comparison of slide DNA with constitutive DNA from buccal epithelium revealed loss of heterozygosity (LOH) with at least one marker in four of 11 leukaemias. Marrow smears offer an attractive archival source of DNA for the detection of microsatellite markers. Applications include detection of LOH or tracing the origin of cell populations, e.g. in bone marrow transplant recipients.