To assess if the quantity of silver-stained nucleolar organizer region (AgNOR) proteins predicts the behavior of actinic keratosis (AK), we performed a standardized AgNOR analysis on 51 cases of AK; in addition, 10 cases of squamous cell (SCC) and 10 cases of basal cell (BCC) carcinomas and 10 normal skin samples were also studied. AgNOR analysis was performed on formalin-fixed and paraffin-embedded sections according to the guidelines of the Committee on AgNOR Quantification (1995), evaluating the mean area (microm(2)) of AgNORs per nucleus (NORA). A highly significant P value (< 0.001) was found in the comparison among NORA values of normal skin (1.869 microm(2); SD + 0.332), AK (3.988 microm(2); SD + 0.914), BCC (3.044 microm(2); SD + 0.254), and SCC (5.286 microm(2); SD + 0.920). In AK, a progressive increase of mean NORA values was observed moving from Stage I (3.161 microm(2); SD + 0.600) to Stage II (3.455 microm(2); SD + 0.562), Stage III (4.360 microm(2); SD + 0.295), and Stage IV (5.168 microm(2); SD + 0.694); highly significant differences (P < 0.001) were noted when Stages I or II were compared with Stage III or Stage IV or between these latter stages. The AgNOR quantity may identify AKs with high proliferative activity and increased tendency to develop into invasive SCC.