ACADIA Pharmaceuticals (ACAD) had the highest return between 2012 and 2013 by a mid-cap US stock, returning 2,171.8%.

ASSET	YEARS	% RETURN
ACADIA Pharmaceuticals (ACAD)	2012-2013	2,171.82%
Vipshop (VIPS)	2012-2013	1,294.6%
Nexstar Broadcasting (NXST)	2012-2013	612.38%
Patrick Industries (PATK)	2012-2013	600.47%
Radian (RDN)	2012-2013	498.18%
Ionis Pharmaceuticals (IONS)	2012-2013	442.04%
American Airlines (AAL)	2012-2013	385.58%
Acadia Healthcare (ACHC)	2012-2013	381.98%
Axos Financial (AX)	2012-2013	381.76%
YY Inc Class A (YY)	2012-2013	378.85%
Sarepta Therapeutics (SRPT)	2012-2013	349.67%
Mr Cooper (COOP)	2012-2013	347.62%
Ligand Pharmaceuticals (LGND)	2012-2013	338.33%
QXO (QXO)	2012-2013	337.5%
Melco Resorts & Entertainment (MLCO)	2012-2013	297.77%
Repligen (RGEN)	2012-2013	294.22%
Mueller Water Products (MWA)	2012-2013	290.49%
Western Alliance Bancorporation (WAL)	2012-2013	276.34%
Saia (SAIA)	2012-2013	271.81%
Archrock (AROC)	2012-2013	265.78%
Avis Budget (CAR)	2012-2013	264.14%
Civitas Resources (CIVI)	2012-2013	243.91%
Grifols SA ADR (GRFS)	2012-2013	242.06%
Trex (TREX)	2012-2013	239.87%
Whirlpool (WHR)	2012-2013	239.58%

Mid-cap stocks are treated as having a market cap between $2B and $10B.