Background: Primary adenocarcinoma of the appendix (PAA) is a rare tumor, and it represents 0.03% of all appendiceal pathology. Diagnosis is made during histopathological study in patients with acute appendicitis. Objective: This study aimed to present our experience in this pathology. Methods and Materials: Retrospective study of all patients with acute appendicitis (AA) at University Hospital La Paz of Madrid, during 47 years (1967–2014). We studied age, gender, preoperative diagnose, surgery, histopathology, evolution and survival rate. Results: There were 44 patients with mean age 60.3 years (32–91) with PAA; 25 (47%) were male. The most common preoperative diagnosis was AA in 29 cases (66%), followed by right lower quadrant plastron in 12 cases (27%). AA coexisted in 12 cases (27%). The first surgery was an appendectomy in 32 patients (73%) and right colectomy in 12 (27%). In 32 patients who had an appendectomy, a right colectomy was done between 2 and 5 weeks after first surgery. 30 cases (68%) were well differentiated mucosecretor adenocarcinoma. In 37 cases, tumoral invasion affected serous layer without lymph node affection. 12-year survival rate was 59%. Mean follow up time was 8.2 years (4 months–32 years). Conclusions: PAA is a rare tumor, and in our series it represented 0.01% of 53,019 appendectomies. Preoperative diagnosis was unusual. Clinical presentation was similar to appendicitis. Elective treatment was right colectomy. Highlights