A novel approach and conceptual ideas are outlined for risk-based navigational guidance of ships using decision support systems in combination with onboard, in-service monitoring systems. The guidance has as the main objective to advise on speed and/or course changes; in particular with focus on ship operations in rough weather. It is strived for to make use of a probabilistic framework considering the mathematical procedures that the guidance relies upon. The paper presents a novel concept which has the possibility to increase the reliability of the provided guidance, although information about on-site sea state parameters not necessarily is in complete agreement with the unknown and true wave parameters, nor may the hydrodynamical models of the vessel give a perfect quantitatively description of the vessel in waves. The paper includes an analysis of full-scale motion measurements and the proposed concept for navigational guidance gives promising results.