Introduction: Kidney stones are a common healthcare problem, and both prevalence and incidence are increasing worldwide. Kidney stone recurrences are frequent. Still, a number of kidney stones pass without symptoms and are not noticed by the patient. Data are limited regarding the frequency of asymptomatic spontaneous stone passage. The aim of the study is to evaluate the frequency of asymptomatic spontaneous stone passage and its predictive factors. Methods: We analyzed the data from the NOSTONE trial, a prospective, multicenter, double-blind, placebo-controlled randomized trial to assess the efficacy of hydrochlorothiazide (HCT) in the prevention of recurrence in patients with recurrent calcium containing kidney stones. We identified all asymptomatic spontaneous stone passages by comparing the total number of kidney stones on low-dose non-intravenous contrast CT imaging at the beginning and at the end of the 3-year follow-up, taking into account the symptomatic spontaneous stone passages and the surgical stone removals. Association of independent variables identified in the data set and the number of asymptomatic spontaneous stone passages using linear regression analyses. Results: A total of 416 patients were randomized and included in the NOSTONE trial, 5 outliers were removed, therefore we analyzed 411 patients. Median follow-up was 35 months (IQR: 29 – 41); median patient age was 49 years (IQR: 39 – 55); 84/411 (20%) patients were female. A total of 476 stone events occurred in 224 of 411 patients (55%); 245/476 (52%) were symptomatic spontaneous stone passages, 73/476 (15%) were surgical stone removals, and 158/476 (33%) were asymptomatic spontaneous stone passages. The median size of asymptomatic stones (2.4 mm; IQR: 1.95-3.4) and the size of symptomatic stones (2.15 mm; IQR: 1.68-2.79) that passed spontaneously were not significantly different (p = 0.366). The number of asymptomatic spontaneous stone passages was significantly influenced by a higher number of stones on CT imaging at randomization (p = 0.001) and a lower total stone volume (p = 0.001). Conclusions: Stone recurrence in recurrent calcium containing kidney stone formers is high. A relevant number of stones, however, pass spontaneously, many of these being asymptomatic (39%). Predictive factors for asymptomatic spontaneous stone passages are the number and the total volume of stones. Our data presented should encourage urologists to counsel patients about the possibility of a conservative approach if kidney stones are present. SOURCE OF Funding: none