Heritage language (HL) proficiency confers developmental benefits; however, the onset of HL loss is observed among many young children from immigrant families. In this longitudinal study, transactional associations between children's HL proficiency and supportive family climate were examined in Chinese immigrant families with pre-school-aged children. Parental warmth, cultural maintenance values, and use of HL support were investigated as aspects of family climate. Measures included observable parent-child interactions and performance-based language proficiency assessments. While parental cultural maintenance values appeared influential, parental behavioral support of HL showed more robust prospective associations with children's HL development. Concurrently, children's earlier HL proficiency predicted subsequent parental behavior; parents whose children had limited HL proficiency decreased their use of HL support 1 later. Implications of the findings are discussed for immigrant parents with young children. (Contains 3 tables and 1 figure.)