['To report a simplified and effective method for substratification of M1 castrate-resistant prostate cancer (CRPC) by correlating progression-free (PFS) and overall survival (OS) with simple quantification of skeletal metastases.', 'In all, 561 men with M1 CRPC were studied longitudinally. Individual bone scan disease burden, quantified by counting bone metastasis number, was correlated with clinical outcome using specific threshold points of 1-4, 5-20 and >20 detectable lesions.', 'Patients with a higher metastasis number had a shorter PFS and OS (hazard ratio [HR] 2.0, 95% confidence interval [CI] 1.7-2.4; P < 0.001). Patients with 1-4 metastases had much better PFS and OS than those with 5-20 metastases. The median PFS and OS in the latter was 10.9 (95% CI 8.4-12.8) and 22.1 (95% CI: 18.5-24.5) months, respectively. PFS and OS for patients with >20 metastases were shorter still [median 5.3 (95% CI 3.4-6.9) months and 13.3 (95% CI 11.3-17.6) months, respectively]. Dichotomising into cohorts with 1-4 and ≥5 metastases, the latter group had considerably poorer PFS [8.4 (95% CI 6.8-10.3) months; P < 0.001) and OS [18.7 (95% CI 17.5-22.1) months; P < 0.001].'] Does quantification of skeletal metastases in castrate-resistant prostate cancer predict progression-free and overall survival?