Cell kinetic parameters were evaluated using the method based on in vivo incorporation of Bromodeoxyuridine (BrdU) and flow cytometric (FCM) analysis in 30 human epidermoid head and neck tumors from oropharynx, oral cavity, rhinopharynx, larynx and lips. BrdU was injected four/six hours before the obtainment of multiple bioptic samples from the tumor tissues. The flow cytometric method was carried out on 70% ethanol fixed cell suspensions based on established protocol for the simultaneous evaluation of DNA content and BrdU uptake using anti-BrdU monoclonal antibodies. We have evaluated the following FCM parameters: DNA ploidy, the degree of DNA aneuploidy (DNA index), Labelling Index (LI), duration of s-phase (Ts) and tumor potential doubling time (Tpot). LI values ranged from 1.5 to 20% with a median value of 10%. The median LI of DNA diploid tumors was 5.4% compared to 14% in DNA aneuploid tumors. Ts values ranged from 8 to 11, the median value being 10 hours. Tpot values ranged from 2 days to 16 days, the median Tpot being 5 days. The large heterogeneity of all these parameters indicates that these tumors may have a different degree of biologic aggressiveness (9). Tpot values did not correlate with DNA ploidy nor with lymph node metastasis status. Tpot values did not correlate in a statistically significant manner with degree of differentiation although shorter Tpot were more frequently observed in moderate or poorly differentiated tumors. Our study shows that the FCM-BrdU technique in vivo is feasible in a clinical setting to evaluate the proliferative behaviour of head and neck tumors, before any specific therapeutic decision is taken after surgery is performed. It is likely that tumors with more aggressive biological behavior, as indicated by LI > 15%, DNA aneuploidy and Tpot < 5 days, may benefit from more aggressive therapies such as accelerated regimeus of radiotherapy and/or other multimodal therapies in respect to tumors with slow growth rate (LI < 15%), DNA diploidy and Tpot > 5 days. So far, however, it still remains to be demonstrated from randomized clinical trials if the knowledge of such individualized cell Kinetic parameters really can help to choose the most effective therapy for every individual patient.