Forty-three squamous cell carcinomas of the head and neck region were in vivo labelled with the thymidine analogue iododeoxyuridine. Combined flow cytometric (FCM) and immunohistochemical (IHC) analysis was performed, and the following parameters calculated: labelling index (LI), S-phase time (TS) and potential tumor doubling time (Tpot). Complete FCM and IHC analyses could successfully be performed in 31 cases, showing a median LI of 13.6% with FCM and 9.1% with IHC. A correlation achieved between LI/FCM and LI/IHC was due to the aneuploid cases, whereas the diploid cases showed no such correlation. Data indicated that Tpot calculated with LI from IHC (Tpot/IHC) might be a prognostic factor, in contrast to Tpot determined using LI/FCM.