Several medical conditions (e.g., brain injury, stroke, progressive neurological diseases) can lead to complete paralysis while largely preserving sensory and cognitive functions and associated brain activation. We investigated whether healthy subjects are able to "write" solely on the basis of voluntary control of the fMRI (BOLD) signal. Using a guided display technique, we show that subjects can learn in less than half an hour to produce reliably any letter of the alphabet in a single trial. To achieve this performance, subjects use three mental strategies to modulate spatio-temporal properties of the fMRI signal in three different brain areas. While the transmitted information (BOLD time courses from regions-of-interest) has been initially decoded offline by human raters, we have recently implemented a fully automatized real-time "brain reading" technique.The developed paradigm and decoding technique might be applied in locked-in patients to let them communicate their wishes and thoughts reliably without extensive pre-training.