Background: Less than 20% of cancers are diagnosed as a result of standard-of-care (SOC) screening in the US. MCED tests may expand screening to more cancers, but the long-term outcomes of MCED test-detected cancers are unknown. DETECT-A was the first large prospective interventional clinical trial to evaluate an MCED blood test. The test used was an early version of CancerSEEK and was evaluated in 9,911 women without history of cancer (Science 369:6499, 2020). CancerSEEK, coupled with diagnostic PET-CT, safely identified cancers including those not detected by SOC screening, the majority of which were localized or regional. This follow-up observational study evaluated longitudinal clinical outcomes of cancers diagnosed as a result of an abnormal CancerSEEK test with a median follow up of 4.4 (IQR:4.1-4.6) years from initial CancerSEEK testing. Methods: Nine cancer types were diagnosed in 26 participants whose cancers were first detected by CancerSEEK. Information on cancer diagnosis, treatment, treatment response, remission status, recurrence, secondary cancer diagnoses, and mortality (cancer-related and all-cause) was extracted from electronic medical records through November 2022. Data collection for living participants took place a median of 3.7 years following cancer diagnosis (IQR: 3.3-3.9) and a median of 4.3 years (IQR: 4.1-4.7) following initial CancerSEEK screen. Results: Fourteen (53.8%) participants underwent surgery; 5 had surgery alone, 9 had surgery with adjuvant and/or neoadjuvant chemotherapy, radiation, or hormone therapy. Twelve of 14 (85.7%) surgically treated participants were in remission as of November 2022, including 10 with localized or regional disease at diagnosis. Among patients whose treatment was non-surgical (11) or unknown (1), 1 (8.3%) was in remission (stage I at diagnosis), 9 (75.0%) were deceased (all stage III or IV at diagnosis), and 2 (16.7%) were in surveillance or ongoing treatment (stage II and stage III at diagnosis). Overall, 13 of 26 (50%) participants were in remission [ovarian (4), thyroid (1), uterus (2), breast (1), colorectal (2), and lung (3)]; 7 of these 13 (54%) had cancers without recommended SOC screening modalities. Eleven of 17 (64.7%) participants with localized or regional disease (stage I, II, or III) were in remission, whereas 2 of 9 (22.2%) with stage IV disease at diagnosis were in remission. Conclusions: CancerSEEK detected cancers earlier in patients who, when treated subsequently with conventional methods, achieved long-term survival. Half of all patients with a CancerSEEK-detected cancer remain cancer-free after treatment >4 years (median) after their initial CancerSEEK test. This includes 7 patients with cancer types for which no SOC screening option exists. Additional biomarkers, new analytic methods and algorithms are being incorporated in the development of the next generation of the MCED test.