臺灣健保醫療費用一直面臨增漲的壓力，固然健保制度規劃之初已預見人口老化將造成費用上漲的結果，人口老化對於醫療費用之影響卻迄無完整的分析，對於人口老化帶來疾病擴張的第二波影響更是未有所聞。本文嘗試從人口老化與醫療費用的形式關係，也就是平均醫療費用的定義下手，解析人口老化對醫療費用的影響。我們將平均醫療費用分解為每日費用、就醫日數、就醫頻率、與人口組成四個組成項，發現人口老化確有穩定推動平均醫療費用向上增漲的作用，影響不大；每日費用略有起伏，呈下降趨勢；就醫頻率起伏較大，卻呈水平趨勢；就醫日數不斷上昇，對醫療費用的影響最大。簡而言之，存活為疾病的必要條件，人死了就不可能患病；在死亡率下降的過程中，中老年人死亡率之下降逐漸替換嬰幼與青年死亡率之下降，成為主要的成份，則死亡率持續下跌涵蘊著疾病盛行率上漲的結果；盛行率上漲的意義是平均每人存活在疾病狀態下的期間增長，就醫的實質需求增加，而醫療體系也能滿足這些需求，自然帶來醫療費用上漲的結果。

Based on the definition of the average NHI medical expenditures, this paper decomposes the average expenditures into four components; namely expenditures per day, days per outpatient visit or hospital stay, visits or stays per year per person, and age composition of the insured population. It is demonstrated that the expenditures per day component declined with small fluctuations during the 1996-2005 period, the visits per year component fluctuated horizontally, the age composition component steadily pushed the average expenditures upward, and the duration of visit was the component most important in bringing the average expenditures upward. A morbidity expansion, combined with the population aging, is proposed as a plausible explanation for the increase in medical expenditures.