Background: A meta-analysis of prospective trials systematically investigated regression of advanced solid tumors in patients receiving placebo or no anticancer therapy to inform on spontaneous regressions. Patient and methods: Arms of randomized controlled trials (RCTs) of metastatic solid tumors receiving placebo or no anti-cancer therapy were used. Statistical analyses were conducted to calculate the overall response rate (ORR) and to detect differentials based on histology, progression at baseline and prior therapies. Results: A total of 7676 patients were evaluable from 61 RCTs evaluating 18 solid tumors. The ORR was 1.95% (95% CI: 1.52-2.48%). There was no significant effect of histology (p = 0.110), baseline progressive disease (p > 0.20) or the line of therapy (p > 0.20) on ORR. Conclusions: Spontaneous regressions are seen across all advanced solid tumors. Some malignancies demonstrated higher rates of spontaneous regressions and may be relatively immunotherapy responsive.