This analysis assessed the effect of lenalidomide on progression-free survival (PFS). Patients with relapsed or refractory multiple myeloma (RRMM) who received lenalidomide plus dexamethasone in the MM-009 and MM-010 trials were pooled and those who had not progressed and were still receiving lenalidomide at 12 months were included. The median follow-up of surviving patients was 48 months. Of 353 patients who received lenalidomide plus dexamethasone, 116 (33%) had not progressed. Overall, 52 patients (45%) had no dose reductions, 25 (22%) had dose reductions ≥12 months and 39 (34%) had dose reductions before 12 months. Patients who had dose reductions ≥12 months had a significantly longer median PFS than those who had reductions before 12 months (P=0.007) or no dose reductions (P=0.039) (not reached vs 28.0 vs 36.8 months, respectively). In a multivariate Cox regression model, dose reduction ≥12 months was an independent predictor of improved PFS (hazard ratio, 0.47; 95% confidence interval, 0.23-0.98) after adjusting for patient characteristics. The data suggest that to achieve maximum PFS benefit, patients with RRMM should be treated for ≥12 months with full-dose lenalidomide plus dexamethasone. Thereafter, patients may benefit from lower-dose continued therapy; prospective studies are needed to confirm these findings.