Described is a lock connection device for medical fluid lines comprising a first connector for coupling a first medical fluid line and a second connector for coupling a second medical fluid line, wherein both connectors are adapted for releasable attachment to each other. The device comprises a first closing member adapted to be releasably attached to the first connector so as to close it, and a second closing member adapted to be releasably attached to the second connector so as to close it. The first and second closing members are configured to attach to each other. If the first closing member is attached to the first connector and the second closing member is attached to the first closing member and the second connector, the first closing member can be detached from the first connector wherein the second closing member remains attached to the first closing member and the second connector.