Objective: To provide an overview of internal organ involvement (IOI) in immunoglobulin G4-related sialadenitis (IgG4-RS) patients, with a focus on the prevalence and clinical features of IOI, the analysis of serum IgG4 levels in patients with or without IOI, and the usefulness of positron emission tomography (PET) for examination of the whole body. Methods: A systematic search was performed using PubMed, CNKI, Wanfang Data and CQVIP databases. Results: A total of 99 articles, including 493 IgG4-RS cases, were analysed in this study. The male-to-female ratio was 1.57:1 and the mean age was 61.67 years. IOI was observed in 71.6% patients, including lesions of the pancreas (38.5%), the biliary system and liver (17.8%), distant lymphadenopathy (20.3%), the respiratory system (15.6%), the urinary system (12.0%) and retroperitoneal fibrosis (11.4%). The lesions could occur homeochronously or metachronously with IgG4-RS. The serum IgG4 levels in the IOI-positive and IOI-negative groups were 1,131 ± 952 mg/dL and 659 ± 843 mg/dL, respectively (P 0.01). The prevalence of IOI and the number of involved internal organs between the PET and the non-PET groups showed no significant difference (P = 0.399 and P = 0.823, respectively), but were significantly higher in the PET group, amongst patients whose first symptom or chief complaint was salivary gland swelling (P = 0.002 and P = 0.001, respectively). Conclusion: IOI is common in IgG4-RS and almost every organ can be affected. High levels of serum IgG4 represent a potential indicator of IOI. Furthermore, PET is a useful tool for evaluation of the whole body. Keywords: IgG4-RD, IgG4-RS, internal organ involvement, serum IgG4 level, PET