Purpose: To review the demographic and clinical features and the success rates of treatment approaches for idiopathic granulomatous mastitis (IGM). Methods: A total of 134 patients diagnosed with IGM pathologically and treated by a single breast surgeon were retrospectively analyzed. Results: The time between the occurrence of symptoms and the last delivery was < 24 months in 25 (23.1%), 24–48 months in 51 (38.1%), and > 48 months in 52 (38.8%). The difference was statistically significant (p = 0.002). Although there was no significant difference, seasonal fluctuations were noticed, with the incidence being slightly higher during late spring and summer. Bilateral disease was present in 10 (7.5%) patients. Seven patients (5.2%) had erythema nodosum. The treatment approaches were conservative in 42, surgical procedures in 48, steroid treatment in 18, and surgical procedure + steroid treatment in 24. The median recovery time with conservative approaches was lower than that with surgical procedure + steroid treatment (p < 0.0001). Recurrence developed in 10 patients (7.7%). Conclusion: Clinical differences were detected among the patients with IGM, and classification of patients by severity is needed to plan the optimal treatment approach. Seasonal fluctuations suggest the possibility of an immunological disease rather than a surgical disease.