Background: Retroperitoneal lymph node dissection (RPLND) is recommended for residual masses > 1cm post-chemotherapy (pc) for nonseminomatous germ cell tumors (NSGCT). There is no reliable predictor for pcRPLND histology and up to 50% will harbour necrosis/fibrosis only, thus rendering a potentially morbid surgery to be of limited therapeutic value. Objective: To evaluate the ability of defined serum microRNA (miRNA) using the ampTSmiR test to predict residual viable NSGCT after chemotherapy. Methods: Serum miRNA levels (miR-371a-3p, miR-373-3p and miR-367-3p) were measured in 82 patients (cohort A = 39, cohort B = 43) treated with orchiectomy, chemotherapy and pcRPLND to predict viable GCT post-chemotherapy. Outcomes, measurements and statistical analysis: miRNA levels were compared to clinical characteristics, serum tumor markers and correlated with presence of viable GCT (vs. teratoma; vs. necrosis/fibrosis). miRNA-discriminative capacity was determined by receiver operating characteristic (ROC) analysis. Results: Serum miRNA were associated with stage at the time of chemotherapy and declined significantly post-chemotherapy. Patients with fibrosis/necrosis and teratoma had a significant decline in all three miRNA levels after chemotherapy, while those with viable disease had very little change. Patients with necrosis/fibrosis demonstrated similar miRNA levels as patients with residual teratoma. miR-371a-3p demonstrated the highest discriminative capacity [area under the curve (AUC) 0.874, CI 95% 0.774 - 0.974 p < 0.0001] for viable disease post chemotherapy. If considering a more relaxed cut-point of 3cm before consideration of pcRPLND, miR-371a-3p correctly stratified all patients with residual retroperitoneal lesions ≤ 3 cm (p= 0.02; 100% sensitivity). Conclusions: Our study is the first to explore a miRNA-based serum test to determine histology in post-chemotherapy residual masses and we demonstrated the value of miR-371a-3p to predict presence of viable GCT. Prospective studies are required to confirm its clinical utility.