OBJECTIVES: To calculate NNTs for panitumumab+FOLFOX4 vs FOLFOX4 alone (PRIME study) to achieve one objective response (OR rate [ORR] analysis) or prevent one disease progression (PFS analysis) or death (OS analysis) at 6/12 or 12/24 months.  METHODS: In PRIME, 1183 patients with previously untreated mCRC were randomised. Using data from an exploratory analysis conducted when 80% of patients had died, NNTs (Bender. Encyclopedia of Biostatistics, Second Edition; Volume 6, pp3752-61) were retrospectively calculated in patients with KRAS/NRAS WT (n=505; exons 2-4 assessed) mCRC overall and in the ECOG performance score 0/1 (n=473) subgroup. ORR and PFS events were defined using RECIST. RESULTS: In the overall KRAS/NRAS WT population, 59.3% vs 45.6% of patients receiving panitumumab+FOLFOX4 vs FOLFOX4 alone, respectively, achieved an OR; NNT(95% CI) in the ORR analysis was 7.3(4.5-19.9). In the ECOG 0/1 subgroup, 61.3% vs 46.4% of those receiving panitumumab+FOLFOX4 vs FOLFOX4 alone, respectively, had an OR; the NNT(95% CI) was 6.7(4.2-16.5). For the PFS overall population analysis, the HR(95% CI) was 0.74(0.62-0.90); HR(95% CI) for the ECOG 0/1 subgroup was 0.72(0.59-0.88).  NNTs(95% CI) in the overall population at 6 and 12 months were 12.8(8.3-34.1) and 9.2(5.7-25.9), respectively; corresponding values for the ECOG 0/1 subgroup were 11.9(7.9-27.8) and 8.4(5.3-20.8). For the OS overall population analysis, the HR(95% CI) was 0.76(0.63-0.93); HR(95% CI) for the ECOG 0/1 subgroup was 0.74(0.60-0.90). NNTs(95% CI) in the overall population at 12 and 24 months were 16.8(10.4-57.1) and 10.4(6.2-37.1), respectively; corresponding values for the ECOG 0/1 subgroup were 16.0(10.3-44.1) and 9.4(5.9-27.2). CONCLUSIONS: In these analyses NNTs for PFS, OS and particularly ORR were low, reflecting the high therapeutic benefit of adding panitumumab to FOLFOX4 in patients with KRAS/NRAS WT mCRC. Limitation by ECOG score 0/1 further lowered NNTs for PFS and OS.