DNA patterns were analysed in 26 GH-producing pituitary adenomas by flow cytometry as well as by microspectrophotometry. Twelve tumours (46%) were diploid according to both methods, whereas 5 tumours (19%) showed aneuploid DNA patterns. Nine tumours were classified differently by the two methods: flow cytometry resulted in diploidy in 2 and aneuploidy in 7 patients, whereas microspectrophotometry showed diploidy in 5 tumours, tetraploidy in 3 and aneuploidy in 1. Methodological limitations may explain the discrepancy in the results obtained by the two methods. However, both the flow cytometry and the microspectrophotometry method show the presence of aneuploid DNA patterns in GH-producing pituitary adenomas despite their benign growth characteristics and the clinically benign course of the disease. This comparative study with two methods measuring DNA content, shows that depending on the criteria used for diploidy-aneuploidy, the freqency of aneuploidy will vary. In this material of 26 GH-producing adenomas, 46% were aneuploid according to flow cytometry and 23% according to microspectrophotometric. However, no correlation to tumour size or GH levels was found with either method when patients with aneuploid and diploid tumours were compared. Therefore, no clinial significance can so far be drawn from these results.