This article describes the development and testing of a research instrument, Families' Importance in Nursing Care-Nurses' Attitudes (FINC-NA), designed to measure nurses' attitudes about the importance of involving families in nursing care. The instrument was inductively developed from a literature review and tested with a sample of Swedish nurses. An item-total correlation and a first principal component analysis were used to validate the final instrument, including a second principal component analysis to analyze dimensionality, and Cronbach's alpha was used to estimate internal consistency. The instrument consists of 26 items and reveals four factors: families as a resource in nursing care, family as a conversational partner, family as a burden, and family as its own resource. Cronbach's alpha was 0.88 for the total instrument and 0.69 to 0.80 for the subscales. The instrument requires further testing with other nurse populations.