Multiple Endocrine Neoplasia type 1 (MEN1) is an autosomal dominant disorder characterised by the combined occurrence of parathyroid, pituitary and pancreatic islet tumours, and is due to mutations of the MEN1 gene, which encodes the tumour suppressor protein menin. MicroRNAs (miRNA) are non-coding single stranded RNAs that post-transcriptionally regulate gene expression. Alterations in miRNA expression, including downregulation of two miRNAs, miR-15a and miR-16-1, have been reported in several tumour types including pituitary adenomas, however their contribution to MEN1-associated tumourigenesis is unknown. In this study we therefore investigated the expression of miR-15a and miR-16-1, in pituitary tumours from a MEN1 mouse model (Men1+/−). We confirm that both miR-15a and miR-16-1 are downregulated in Men1+/− pituitary tumours compared to normal, wild-type pituitaries (2.3 and 2.1-fold, respectively; P<0.003). Furthermore, we investigated the expression of the miR-15a/miR-16-1 target, Ccnd1, and found Ccnd1 was significantly upregulated in Men1+/− pituitary tumours, compared to wild-type pituitaries (2.5-fold; P<0.005); Ccnd1 expression was also significantly inversely correlated with the expression of miR-15a and miR-16-1 (R2=0.81 and 0.78, respectively; P<0.005). To confirm that cyclin D1 upregulation was a direct result of miR-15a/miR-16-1 loss, we inhibited the activity of miR-15a and miR-16-1 in a human cell-line (HeLa), using antagomirs. Reduction of miR-15a and miR-16-1 significantly increased cyclin D1 expression (3.2 and 3.8-fold, respectively; P<0.01); but had no effect on menin expression. To further examine the relationship between miR-15a/miR-16-1 and menin we silenced menin, using siRNA, in HeLa cells. Silencing menin had no effect on miR-16-1 expression but significantly decreased miR-15a expression (twofold, P<0.008). Thus, our study demonstrates that expression of the cyclin D1 suppressors, miR-15a and miR-16-1, is decreased in Men1+/− pituitary tumours, and that downregulation of miR-15a could be a direct result of menin loss.