Accurate diagnosis of systemic amyloidosis is necessary both for assessing the prognosis and for delineating the appropriate treatment. It is based on histologic evidence of amyloid deposits and characterization of the amyloidogenic protein. We prospectively evaluated the diagnostic performance of immunoelectron microscopy (IEM) of abdominal fat aspirates from 745 consecutive patients with suspected systemic amyloidoses. All cases were extensively investigated with clinical and laboratory data, with a follow-up of at least 18 months. The 423 (56.8%) cases with confirmed systemic forms were used to estimate the diagnostic performance of IEM. Compared with Congo-red–based light microscopy, IEM was equally sensitive (75% to 80%) but significantly more specific (100% vs 80%; P < .001). In amyloid light-chain (AL) amyloidosis, κ cases were more difficult to diagnose (sensitivity 71%), whereas the analysis of abdominal aspirate was informative in only 40% of patients with transthyretin amyloidosis. We found a high prevalence (20%) of a monoclonal component in patients with non-AL amyloidosis, highlighting the risk of misdiagnosis and the need for unequivocal amyloid typing. Notably, IEM identified correctly the specific form of amyloidosis in >99% of the cases. IEM of abdominal fat aspirates is an effective tool in the routine diagnosis of systemic amyloidoses. •The first wide, prospective report on the role of IEM in the differential diagnosis of systemic amyloidosis.•IEM allows for the correct characterization of the amyloid protein in virtually all cases and represents a viable alternative to mass spectrometry.