Hecla Mining (HL) had the highest return between 2001 and 2003 by a mid-cap US stock, returning 1,373.8%.

ASSET	YEARS	% RETURN
Hecla Mining (HL)	2001-2003	1,373.85%
Companhia Siderurgica Nacional ADR (SID)	2001-2003	1,133.98%
FirstCash (FCFS)	2001-2003	1,024.06%
Merit Medical Systems (MMSI)	2001-2003	999.26%
Acadia Healthcare (ACHC)	2001-2003	969.23%
Urban Outfitters (URBN)	2001-2003	871.83%
Hudbay Minerals (HBM)	2001-2003	825.08%
Option Care Health (OPCH)	2001-2003	765.23%
SSR Mining (SSRM)	2001-2003	720.94%
Bank Ozk (OZK)	2001-2003	717.55%
Dorman Products (DORM)	2001-2003	699.87%
CarMax (KMX)	2001-2003	685.5%
Middleby (MIDD)	2001-2003	580.36%
Darling Ingredients (DAR)	2001-2003	530.86%
Rush Enterprises B (RUSHB)	2001-2003	528.43%
Cal-Maine Foods (CALM)	2001-2003	521.43%
Thor Industries (THO)	2001-2003	474.66%
Denison Mines (DNN)	2001-2003	471.36%
Light & Wonder (LNW)	2001-2003	458.22%
FTI Consulting (FCN)	2001-2003	425.84%
DRDGOLD Limited ADR (DRD)	2001-2003	407.23%
Itron (ITRI)	2001-2003	397.9%
LCI Industries (LCII)	2001-2003	394.2%
Enerpac Tool (EPAC)	2001-2003	383.16%
Silgan (SLGN)	2001-2003	372.21%

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