RATIONALE AND OBJECTIVES.Laser-induced interstitial thermotherapy (LITT) may become an attractive modality for minimally invasive tumor therapy. Magnetic resonance imaging (MRI) could be used to assist this procedure. METHODS.A Tl-weighted turbo fast low-angle shot (FLASH) sequence for on-line monitoring of the laserinfluenced region (liver, muscle) was investigated. Sequence parameters were optimized for maximal image contrast. Magnetic resonance imaging-controlled LITT was performed in vitro, in vivo (rabbits), and in 8 human investigations (6 patients). Special laser applicators were used to establish a uniform laser light distribution. RESULTS.With the MRI sequence used, the LITT region is visualized as a bright area outlined by a dark border. This dark border corresponds to an isotherm of 45 ± 2 °C depending on the sequence parameters used. CONCLUSION.With the Tl-weighted turbo-FLASH sequence, MRI can be used for on-line monitoring of interstitial laserinduced thermotherapy in moving organs.