The aim of this study was to clarify the incidences of and the risk factors for severe retinopathy requiring photocoagulation therapy and albuminuria in Japanese patients with childhood-onset type 1 diabetes mellitus. A total of 756 patients from a cohort study by the Japanese Study Group of Insulin Therapy for Childhood and Adolescent Diabetes were included in the study. Patients were registered in 1995 or 2000, and HbAwas measured every 4 months and analyzed in central hospital for an average of 6 years. The presence of severe retinopathy requiring laser photocoagulation and the presence of albuminuria was checked for during the period 2010-2011. During a median of 18 (range: 15-21) years, 34 out of 756 patients underwent laser photocoagulation and 57 out of 605 patients developed albuminuria. A Cox proportional hazards model showed that the risk of severe retinopathy requiring laser photocoagulation increased by 1.15 (95% confidence interval [CI] 1.03-1.29,  = 0.012) with each increase of a year in the age at onset, by 4.03 (95% CI 1.20-13.5,  = 0.024) in females, and by 2.05 (95% CI 1.69-2.49,  < 0.0001) with each increase of 1% in HbA. The risk of albuminuria increased significantly, by 1.09 (95% CI 1.01-1.18  = 0.037), with each increase of a year in the age at onset and by 2.38 (95% CI 1.93-2.97  < 0.0001) with each increase of 1% in HbA. In Japanese patients with childhood-onset type 1 diabetes, older age at the onset of diabetes, female rather than male gender, and higher HbA were found to increase the risk of requiring photocoagulation. No patients with HbA < 7.5% developed severe retinopathy requiring photocoagulation therapy. The risk of developing albuminuria increased with age at onset of diabetes and HbA. Female gender was a strong risk factor for severe retinopathy requiring photocoagulation, but not for albuminuria.