Although a complete clinical remission can often be achieved with chemotherapy for patients with leukaemia and lymphoma, relapses still occur. Residual tumour cells probably have survived therapy and account for subsequent disease relapse. The sensitivity of conventioned ways of detecting residual tumour cells, such as morphological studies, immunophenotyping, and cytogenetics, is only about 1% to 5% and may be inadequate. Polymerase chain reaction technology has provided a simple and highly sensitive means for the detection of minimal residual disease. The technology has been successfully applied to study biopsy samples obtained from patients with leukaemia and lymphpma. Its clinical usefulness, however, requires further evaluation by prospective clinical studies.