Knowledge on when, where and how Public Participation Geographic Information Systems (PPGIS) influence planning decisions has been spotty and incomplete. To fill the gap, we assessed the influence of public land use preferences, obtained through PPGIS, on selected land use planning cases in Finland and Poland. The selected cases ranged in scale from a neighbourhood to an entire city and involved two types of online PPGIS tools: an interactive map-based questionnaire and a map-based discussion platform. The assessment was based on a qualitative framework organizing potential drivers of the planning process into convening, process and outcome aspects. The assessments results show high number and diversity of participants, the use of PPGIS in the beginning of the planning process, and the quality of PPGIS data representing public land use preferences may influence the content of planning documents. Conversely, a mismatch between plan scope and citizen concerns, legal framework constraints, lack of coordination between overlapping participatory and decision-making processes, and low representativeness of PPGIS data diminish the influence of PPGIS on planning decisions.