Astrana Health (ASTH) had the highest return between 2008 and 2009 by a mid-cap US stock, returning 21,112.1%.

ASSET	YEARS	% RETURN
Astrana Health (ASTH)	2008-2009	21,112.12%
Quantum Computing (QUBT)	2008-2009	820%
Amneal Pharmaceuticals, Inc Class A Common Stock (AMRX)	2008-2009	444.4%
Aura Minerals Inc Common Shares (AUGO)	2008-2009	299.83%
Axogen (AXGN)	2008-2009	209.22%
Pegasystems (PEGA)	2008-2009	190.24%
Eldorado Gold (EGO)	2008-2009	137.35%
Trex (TREX)	2008-2009	126.85%
Boyd Group Services (BGSI)	2008-2009	126.31%
Silvercorp Metals (SVM)	2008-2009	116.91%
NewMarket (NEU)	2008-2009	115.09%
Steven Madden (SHOO)	2008-2009	106.51%
Credit Acceptance (CACC)	2008-2009	100.76%
Veeco Instruments (VECO)	2008-2009	98.56%
Imax (IMAX)	2008-2009	95.74%
IAMGold (IAG)	2008-2009	93.6%
Grand Canyon Education (LOPE)	2008-2009	90.1%
Central Garden & Pet Company A (CENTA)	2008-2009	87.9%
ICL Israel Chemicals (ICL)	2008-2009	86.26%
Central Garden & Pet (CENT)	2008-2009	84.48%
ePlus (PLUS)	2008-2009	82.43%
Northern Oil & Gas (NOG)	2008-2009	82.15%
Baytex Energy (BTE)	2008-2009	77.8%
National Beverage (FIZZ)	2008-2009	73.25%
Celsius (CELH)	2008-2009	58.14%

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