This study reports the characteristics of communication between a nurse and surrounding persons, especially patients and other nurses, in medical wards. The 21 nurses were equipped with IC recorders during time studies of nursing workloads, and all of their conversations were recorded and transcribed. Discourse analyses focused on dialogue for sharing risk information in medical treatments. Several cases showed that ambiguous or inadequate utterances frequently emerged in dialogue among nurses, which showed some potentiality of serious errors in medical care or communication. In other hands, some cases showed effective risk sharing, with open questions which facilitated mutual risk cognition and constructed a new context of dialogue for risk management. These results suggests that risk sharing communication, as problem solving based on collaborative cognition, could be one key concept for medical safety.