Time based prospective memory describes the ability to remember to execute an intention at a predefined future time point while being engaged in an ongoing task. This study addresses the question of how increasing the difficulty of the ongoing task affects prospective memory performance. To investigate this, participants are presented with a word-picture-matching task as an ongoing task. We show 2, 4 or 6 words at different screen positions (depending on the assigned difficulty condition). One of these positions is highlighted after word presentation. Participants have to decide whether the word at this position matches the picture that is presented afterwards. Participants perform this task for 40 minutes. As a prospective memory intention, participants have to remember to press the space bar and type-in a word every 5 minutes. Participants can call a screen clock by pressing an additional key at any time while they perform the task. Data collection is still ongoing but preliminary analyses suggest that prospective memory, clock checking and ongoing task performance were negatively affected by higher ongoing task difficulty manipulations. Results will be discussed in the context of time-based prospective memory theories.