Abstract: A boring tool for boring cylinder bores (6), with a tool holder (8) and a tool head (4) with a cutting edge carrier for at least one cutting edge (3) and with a rotary drive for driving the at least one cutting edge (3). The cutting edge carrier is provided with a first, preferably hydraulic, adjusting device for the adjustment of a variable cutting radius of the cutting edge (3), a ring module for first support rails (1), which concentrically surrounds the tool head (4), a second adjusting device for the variable radial adjustment of the supporting radius of the first support rails (1), and a third adjusting device for variable radial adjustment of a supporting radius of second support rails (2). The tool head (4) is axially moveable within the ring module. A method for boring cylinder bores (6) involving the use of the boring tool is disclosed as well.