BACKGROUND: The time between onset of symptoms of insulinoma to diagnosis ranges from 10 days to more than 20 years. To help physicians make an earlier diagnosis, we defined the clinical, imaging and paraclinical characteristics of insulinoma in cases from seven referral hospitals in Iran over two decades. METHODS: The medical records of 68 cases with biochemical or histological evidences of insulinoma were reviewed. RESULTS: More males were affected (53%). The mean age at diagnosis was 39 +/- 15.3 years. The mean duration of symptoms was 39.9 +/- 59.3 months. Eighty-four percent of patients had been initially misdiagnosed as cereberovascular accident (CVA), epilepsy, conversion disorder, and others). Neuroadrenergic symptoms were observed in 89.6% and and neuroglycopenic symptoms in 97% of patients. Mean diameter of tumours was 2.9 cm (range, 1 cm to 8.5 cm). Of 52 pathologically confirmed cases of insulinoma, 43 tumours (87.8%) were single and 49 (94.2%) were benign. Fifty-five patients had undergone surgery, with a successful outcome in 44 (80%). CONCLUSION: The high incidence of neuroglycopenic symptoms suggest the clinical impression of insulinoma when patients present with a suggestive clinical syndrome. The clinical impression is essential to decrease the frequent delay in the diagnosis of insulinoma.