Cell proliferation is tightly coordinated with cell growth. The oncosuppressor proteins pRb and p53 may exert a key role in coupling growth and proliferation by controlling both ribosome biogenesis and cell cycle progression. In the present study we evaluated the relationship between the pRb and p53 status and rRNA transcriptional activity in histological sections of 343 human primary breast carcinomas. Ribosomal biogenesis was quantified by morphometric analysis of silver-stained interphase nucleolar organizer regions (AgNORs). pRb and p53 status was assessed by immunohistochemistry. Twenty-four tumors were considered to be pRb deleted, 260 tumors showed a phosphorylated-pRb labeling index (LI) up to 25%, and 55 tumors an LI >25%. Tumors with deleted pRb or phosphorylated-pRb-LI > or =25% were characterized by significantly greater mean AgNOR area values than those with unaltered pRb (p<0.001). In the 71 tumors with mutated p53 the NOR area mean value was greater than in the 272 tumors with normal p53 (p<0.001). Our results demonstrate, for the first time in vivo, that pRb and p53 status is related to the ribosome biogenesis rate and suggest that in tumors with altered pRb and p53 function the up-regulation of rRNA synthesis may always assure an adequate growth to cancer cells with uncontrolled cell cycle progression.