Novel anti-myeloma agents have improved patient response rates, which are historically based on reductions of the M-protein. These methods can be inaccurate for quantifying M-proteins at low concentrations. We compared the consistency and clinical impact of response assignment by electrophoretic and heavy+light chain (HLC) immunoassays post-consolidation in 463 newly diagnosed patients. The two methods gave similar assignments in patients with partial (PR; 79% agreement) or complete response (≥CR; 92%). However, in patients achieving very good PR (VGPR) there was poor concordance between methods (45%). Median progression-free survival (PFS) for standard VGPR patients was 34.5 months; HLC responses stratified these patients further into PR, VGPR and ≥ CR, with median PFS of 21.3, 28.9 months and not reached, respectively; P < 0.001. At this time, abnormal HLC ratios had better concordance with multiparametric flow cytometry (sensitivity 10−4) (37 and 34% positive, respectively), compared to immunofixation (62% positive). In addition, HLC-pair suppression was identified in 38% of patients and associated with shorter PFS (30.6 months vs not reached; P < 0.001). We conclude that HLC monitoring could augment electrophoretic assessments in patients achieving VGPR. The prognostic significance of HLC responses might partly depend on the patients' ability to recover their immune system, as determined by normalisation of HLC measurements.