Background: Lanreotide autogel 120mg (lanreotide) was approved in Japan in July 2017 for the treatment of gastroenteropancreatic neuroendocrine tumors (NETs) based on the results from an international phase III study (CLARINET study) and a Japanese single-arm phase II study. The CLARINET extension study demonstrated long-term efficacy and safety of lanreotide treatment; however, no Japanese patients were included in the study. To describe the safety and efficacy of long-term lanreotide treatment for NETs in Japanese patients, the final analysis of the Japanese phase II study and its extension study was performed. Methods: This open-label extension study (Study 002, JapicCTI-142698) enrolled patients who completed 48 weeks of treatment with 4-weekly subcutaneous lanreotide in an open-label phase II study in Japanese patients with NETs (Study 001, JapicCTI-132375). Study 002 had the same design as Study 001. Results: Of the 32 patients who started treatment with lanreotide, 17 completed Study 001 and enrolled in Study 002. In the safety analysis set (n = 17), 16 patients (94.1%) developed adverse events (AEs) that were considered drug-related (adverse drug reactions; ADRs), most commonly faeces pale (n = 5; 29.4%), injection site induration (n = 4; 23.5%), flatulence and diabetes mellitus (both n = 3; 17.6% each). No patient permanently discontinued lanreotide or died as a result of an AE. Four patients had a total of eight serious AEs; of these, two events of bile duct stones were considered to be ADRs. In the efficacy analysis set (n = 28), the median lanreotide exposure over Studies 001 and 002 was 52.7 weeks (range: 12–181 weeks). The best overall response was partial response in 2 patients (7.1%; reached at 60 and 108 weeks), stable disease in 20 patients (71.4%) and progressive disease in 6 patients (21.4%). Conclusions: No unexpected serious AEs developed during prolonged use of lanreotide. Lanreotide was effective over long-term treatment in Japanese patients with NETs. Clinical trial information: JapicCTI-132375, JapicCTI-142698.