Introduction: IgG4-related disease (IgG4-RD) is a clinical entity characterized by multi-organ lymphoplasmacytic infiltration with IgG4 positive plasma cells that results in fibrosis. Type I autoimmune pancreatitis (AIP), one of the manifestations of IgG4-RD, is a rare cause of acute pancreatitis and it has different prognostic and therapeutic implications from other etiologies of pancreatitis. Case Description/Methods: A 27-year-old morbidly obese male presented with nausea, bilious emesis, epigastric pain and non-bloody diarrhea for 4 weeks. Exam revealed papules throughout the body, lower extremity edema and brown stool on rectal exam. Patient had severe anemia (Hg 4.6 g/dL), hyperkalemia (6.6 mmol/L), uremia (BUN 247 mg/dL), elevated creatinine (33.04 mg/dL) and lipase (1,164 U/L) levels. He also had elevated TSH (7.37 uIU/mL), decreased free T4 (0.61 ng/dL) and mildly elevated thyroid peroxidase antibody level (43.6 IU/mL). Alkaline phosphatase, total bilirubin and transaminases were normal.US and CT abdomen showed cholelithiasis without intra/extra hepatic ductal dilation and acute pancreatitis, respectively. In the absence of biliary tract obstruction, alcohol use and with normal triglyceride levels, AIP was suspected. IgG level was elevated (2305 mg/dL) with elevated IgG4 subclass (229 mg/dL) supporting the diagnosis of type 1 AIP. Diagnosis was confirmed with kidney biopsy showing diffuse TIN with lymphoplasmacytic cells positive for IgG4; other autoimmune work up including C3, C4, ANA, ds-DNA, ANCAs was negative. The patient improved with Methylprednisolone and Rituximab therapy. He was referred for kidney transplant and continues to require hemodialysis. Discussion: IgG4-RD is an autoimmune disease with multi-organ involvement and eventual fibrosis.Type I AIP can be diagnosed based on the presence of HISORt criteria; these include histologic, imaging, serologic, other organ involvement and response to glucocorticoid therapy. In our patient, elevated IgG4 level, renal biopsy and response to steroids confirmed the etiology of Type I AIP. Organs affected included kidneys (TIN), thyroid gland (Hashimoto’s thyroiditis), pancreas (AIP type I), lymphatics and skin. This case not only serves an important purpose of demonstrating the heterogeneity of IgG4-RD, but also, stresses the importance of appropriate diagnosis because AIP often can be confused with pancreatic malignancy, which, if incorrectly diagnosed, would alter the management significantly.Figure 1.: Markedly Increased Interstitial Plasma Cells Seen on H&E stain (400x) on Renal Biopsy.Figure 2.: Immunohistochemistry Stain of Renal Biopsy Staining Positive for IgG 4.