Nusinersen is an antisense oligonucleotide for the treatment of spinal muscular atrophy (SMA). A post-marketing surveillance (PMS) has been ongoing (August 2017-August 2025) in all patients in Japan who received intrathecal nusinersen in real-world clinical settings. We report the interim analysis results of safety and effectiveness. This interim analysis was conducted using data collected from 401 patients whose case report forms were obtained at least once by May 30, 2020. Collected data included patient demographics and adverse events (AEs) for safety, and motor function assessments and Clinical Global Impressions of Improvement (CGI-I) for effectiveness. All 401 patients were diagnosed with SMA and were included in the safety and effectiveness analysis (infantile-onset SMA [n = 126, 31.4%], later-onset SMA [n = 275, 68.6%]). The median duration of treatment was 330 days (range 1-823 days). The incidence proportion of AEs was 31.7% (37.3% in infantile-onset SMA and 29.1% in later-onset SMA). The most common AEs were headache (4.5%), pyrexia (4.2%), and pneumonia (3.7%). The incidence proportion of serious AEs was 11.5%. Nusinersen improved motor function scores and was assessed as "effective" based on CGI-I in 99.7-100% of patients. This interim analysis of the PMS in Japanese patients treated with nusinersen found no new safety concerns, with the type of AEs consistent with the expected safety profile. The benefit-risk balance of nusinersen treatment remains favorable.