Although germ cell cancer (GCC) is rare in the general population, it represents the most frequent malignancy in young Caucasian males, predominantly of the testis, accounting for 60% of all malignancies in these males between 20 and 40 years of age [
1]. Its incidence is increasing [
2] and long-term effects of systemic treatment are evident [
3]. Therefore, efforts are being made to diagnose this disease at an early stage. Clinically and histologically, TGCC are divided into two main subtypes, seminoma (SE) and non-seminoma (NS), both derived from pre-invasive germ cell neoplasia in situ (GCNIS) which, in turn, originates from a transformed primordial germ cell or gonocyte [
4,
5]. NS can be heterogeneous, consisting of the histologic variants embryonal carcinoma (EC), yolk sac tumor (YST), choriocarcinoma (CH) and teratoma (TE) [
1,
4,
6‐
8]. Although the 5-year survival rate of TGCC patients exceeds 96%, patients have a long term risk of developing a second primary TGCC or of progression of the disease [
9]. Therefore, they undergo intensive and long-term follow-up after initial diagnosis, including extensive physical examination and monitoring of the levels of the conventional serum protein markers B-HCG, AFP and to a lesser extent LDH, due to its low specificity [
10]. Serum biomarker positivity relates to the histology of the primary (and possibly metastasized) tumor. As such, B-HCG is predominantly related to the presence of a CH component and AFP to a YST component, whereas they show little sensitivity for the detection of SE and the NS stem cell component EC. This explains why a substantial percentage of TGCC patients, predominantly without CH and YST components, can be serum biomarker negative at initial diagnosis or become negative during follow-up, respectively. In addition, false positive findings can be obtained, in case of B-HCG due to other malignancies, chemotherapy or marijuana use and in case of AFP due to chronic liver disease, liver damage or liver cancer [
11,
12]. Therefore, there is a clinical need for biomarkers with a better sensitivity, especially related to (pure) SE and EC, for primary tumor or metastasis detection (reviewed in [
13]). The advent of techniques such as microRNA (miR) profiling has allowed the identification a number of small non-coding RNAs specifically expressed in (T)GCC [
14,
15]. In particular, it has been found that the expression of miR371a-3p, miR-372-3p, miR-373-3p and the miR-302a-d/367-3p cluster is characteristic for (T)GCC as well as embryonic tissues, excluding fully somatic differentiated (TE) tissues. These miRs have been reported to serve as circulating molecular serum biomarkers in patients with testicular and intracranial GCC [
14‐
20]. It has been found that miR-371a-3p outperforms the other miRs in the detection of primary GCCs, the reason of which is still unexplained [
18,
19,
21]. So far, most studies have focused on the value of these biomarkers in primary diagnosis and during chemotherapy [
22], but not yet extensively during follow-up. In the current proof-of-concept study, we examined the levels of miR-371a-3p, miR-373-3p and miR-367-3p in serum samples of a follow-up series of selected TGCC patients ranging from the time of primary diagnosis to relapse and complete remission. Our results confirm the power of elevated serum miR-371a-3p, 373-3p and 367-3p levels for primary TGCC diagnosis and show the efficacy of the ampTSmiR test for TGCT relapse detection, outperforming conventional serum biomarkers.