question: INTRODUCTION We examined the safety and effectiveness of interval laparoscopic appendectomy (ILA) for adult appendiceal mass (AAM) and then used a novel white blood cell (WBC) parameter to identify the types of AAM cases for which nonsurgical treatment followed by ILA is effective. METHODS We reviewed the cases of 956 patients who had undergone appendectomy between April 2012 and March 2018 at our facility. Of these patients, 49 had AAM, including 34 who underwent ILA. We examined the safety and effectiveness of ILA by comparing it with laparoscopic appendectomy (LA); specifically, the 34 cases treated with ILA were compared with 477 cases of adult uncomplicated appendicitis treated with LA from the same patient cohort. We then examined the factors associated with not successfully completing the planned nonsurgical treatment and interval before ILA. Patient demographics and clinical variables were reviewed. RESULTS Patients who had undergone ILA had longer operative times than those who had undergone LA (P = .0059), but they also had shorter postoperative hospital stays (P < .001). There were no significant differences in other perioperative variables. Multivariate analysis showed that a ratio of WBC count on day 3 and day 0 from the start of nonsurgical treatment (WBC day3/day0) of 0.906 or higher was significantly associated with not completing the nonsurgical treatment and interval before ILA (P = .045). CONCLUSION A comparison of the procedures found that ILA for AAM was almost as safe and effective as LA for adult uncomplicated appendicitis. The WBC day3/day0 ratio can be an objective parameter to assess the effectiveness of the nonsurgical treatment before ILA earlier in the course of treatment.