Introduction We describe a 39-year-old female patient who debuts with reactive extreme thrombocytosis and anti-neutrophil cytoplasmic autoantibodies (myeloperoxidase) as an uncommon onset presentation of ANCA-associated vasculitis. This unique case constitutes the first documented case report of its kind in an adult patient.Case Presentation A female adult woman with a 2-month history of constitutional symptoms and polyarticular pain. Laboratory findings revealed extreme thrombocytosis, later with impaired renal function and a renal biopsy showing rapidly progressive pauci-immune crescentic glomerulonephritis (CGN) compatible with microscopic polyangiitis, and positive anti-neutrophil cytoplasmic autoantibodies (myeloperoxidase). Leading to the diagnosis of ANCA-associated vasculitis, specifically microscopic polyangiitis (MPA). Treatment with corticoids pulses and cyclophosphamide resulted in a satisfactory clinical response, marked by the improvement of thrombocytosis and renal function.Conclusions The coexistence of extreme thrombocytosis and CGN is an unusual presentation at onset of ANCA-associated vasculitis MPA type, thereby the diagnosis process is challenging. The concomitance of both conditions represents a severe inflammatory state that needs to be rapidly solved to avoid complications.