Lenalidomide (LD) was reported to increase the risk of thromboembolism when it was used along with dexamethasone (DEX). Prophylactic administration of antithrombotic drugs against thromboembolism has been recommended for proper use of LD, but none of the recommendation is stated in the package insert. The purpose of this study was to elucidate the usage of acetylsalicylic acid (ASA) for lenalidomide medication in patients withmultiple myeloma. We used the MDV analyzer to investigate clinical data retrospectively. The investigation period was from October 1, 2016 to September 30, 2017. Subjects were outpatients aged 20 years or older who were recorded in clinical data as multiple myeloma. There were 7,590 outpatients with multiple myeloma. They were divided into 4 groups by the combined use situation of LD and DEX: LD/DEX non-use group (n＝5,462), DEX alone group (n＝632),LD alone group (n＝203), and LD/DEX together group (n＝1,293), respectively. The prevalence rate of thromboembolism was 7.3% in the DEX alone group and 16.9% in the LD/DEX together group (p＜0.0001). Among the LD/DEX together group, ASA was prescribed at 63.6% in the group without thromboembolism (n＝1,074). The prevalence rate of thromboembolism was higher in the LD/DEX combined group than in the DEX alone group. Considering these findings, risk management for thromboembolism caused by administration of antithrombotic drugs should be considered. It is necessary to create more evidence concerning the necessity of administration of antithrombotic drug in combination with LD/DEX medication.