Research on school improvement tends to assume that school improvement is a school-wide process. Nonetheless, some researchers have also proposed that secondary schools are comprised of subcultures centered on subject area departments. It has further been suggested that variations in the sociocultural organization of subject departments could produce differential results in the learning outcomes of students. To date, however, few empirical studies have examined the leverage offered by this perspective. The current study addresses this gap in the literature by analyzing changes in the learning outcomes of subject departments over a 3-year period as compared with patterns of whole school improvement in 47 Hong Kong secondary schools. The results support the proposition that theory, research, and practice on secondary schools will benefit from viewing their improvement from a perspective that takes into account the diversity of department-level conditions and learning outcomes.