The British Isles lay at a palaeolatitude of 4°S during the Early Carboniferous (Courceyan–Arundian) period. This paper examines fossil gymnosperm wood from ten localities in western Ireland and southern Scotland in order to analyse palaeoclimate. Fifty-two percent of the 77 fossil wood specimens studied exhibit growth rings that possess subtle, discontinuous ring boundaries and ring increments of narrow but variable width. These growth rings are qualitatively and quantitatively analysed, and are shown to bear a close similarity to growth rings in modern araucarian conifer woods; these araucarian growth rings are formed in response to tropical rainfall seasonality linked to monsoonal circulation. The findings of this study therefore support earlier palaeoclimatic interpretations, based on sedimentological evidence, which suggest that the British Isles experienced a monsoonal climate during the Early Carboniferous (Courceyan–Arundian) period.