Lung carcinomas are characterised by considerable histological variation within the tumour. The possible effects of this morphological heterogeneity on the estimation of tumour ploidy were investigated. Multiple tissue blocks were systematically taken from 20 lung tumours and analysed by flow cytometry. The routine, archival paraffin wax embedded diagnostic blocks from these cases were also analysed. Nineteen (95%) of the tumours were shown to contain aneuploid stemlines by systematic sampling, but if only one of these systematic tissue blocks had been taken from each case the incidence of DNA aneuploidy could have been as low as 45%. Only 15 (75%) tumours were aneuploid when all the routine archival blocks were analysed, but by specifically selecting tumour areas from the archival material the accuracy of this method was increased to 90%. It is concluded that tumour sampling methods are of primary importance in assessing the DNA content of lung tumours. Routine paraffin wax embedded archival tissue provides a suitable source of material for this purpose, provided that "turnover" selection is carried out.