The concept of suffering is discussed among those who are cognitively aware and verbally capable to express their suffering. Due to immaturity, preterm infants' abilities to express suffering are limited. Relieving suffering is an ethical and juridical demand of good nursing care. The purpose of this study is to describe nurses' perceptions of the suffering of preterm infants. A descriptive qualitative approach was selected. Data were collected from essays written by nurses (n = 19) working in the neonatal intensive care unit. Inductive content analysis guided by the research question was performed. The nurses described individually determined suffering of the preterm infants according to four categories: suffering ruled by maturation, existence of suffering, individual threshold of suffering and interpreting the cues of suffering. Suffering of preterm infants is manifested by population-specific features, emphasising the need to develop sensitive interventions for relieving their suffering.