We validated and compared the prognostic value of the proliferation marker phosphohistone H3 (PPH3) with classical variables in 241 T(1-2)N(0)M(0) breast cancer patients less than 71 years old with long-term follow-up (median 117 months) and without adjuvant treatment. PPH3 was measured by automated digital image analysis. Thirty-seven patients (15%) developed distant metastases and 29 (12%) died. The previously established PPH3 prognostic threshold H3 <13 (n = 157; 65% of all cases) vs. >or=13 (n = 84; 35% of all cases) was the strongest prognostic threshold exceeding all other characteristics, with 10-year recurrence-free survival of distant metastases of 96 and 64%, respectively (P = < 0.0001, hazard ratio = 7.8, 95% confidence interval = 3.4-17.9). PPH3 is robust as it showed high inter-observer reproducibility and was prognostic over wide range of thresholds around 13 and is the strongest prognostic variable in invasive node-negative breast cancer patients less than 71 years old.