This paper examines the use of regional languages and dialects of Dutch in the Netherlands, and the relation of such to the Dutch language proficiency of primary school pupils. The data from the large-scale primary education PRIMA cohort study are analysed. A total of nearly 35,000 pupils participated in five measurement rounds covering the period 1995 through 2003. Insight is thus provided into the most recent situation and the developments during this period. Two developments stood out in particular. A considerable intergenerational decrease in the use of regional languages and dialects within families, on the one hand, and across years, on the other hand. Only a weak relation between the use of regional languages and dialects and Dutch language proficiency is detected, moreover. The conclusions do not hold for each and every regional language and dialect studied here. The position of Limburgish proved interesting in particular.