Advanced prostate cancer is often treated with anti-androgens which target the androgen receptor (AR) on which the growth of the tumour depends. Prostate cancer often develops anti-androgen resistance via a plethora of mechanisms, many of which are unknown, but it is thought that AR upregulation or AR ligand binding site mutations, may be responsible. Here we describe the production of cell lines based on LNCaP and VCaP, with acquired resistance to the clinically relevant anti-androgens, bicalutamide and enzalutamide. In these resistant cells, we observed, via RNA-seq, that new variants in the 3’UTR of the AR mRNA were detectable and that the levels were increased both with anti-androgen treatment and with hormonal starvation. Around 20% of AR transcripts showed a 3kb deletion within the 6.7kb 3’UTR sequence. Actinomycin D and luciferase fusion studies indicated that this shorter mRNA variant was inherently more stable in anti-androgen resistant cell lines. Of additional interest was that the AR UTR variant could be detected in the sera of prostate cancer patients in a cohort of serum samples collected from patients of Gleason grades 6-10, with an increasing level correlated to increasing grade. We hypothesise that the shorter AR UTR variant is a survival adaptation to low hormone levels and/or anti-androgen treatment in these cells, where a more stable mRNA may allow higher levels of AR expression under these conditions.