We report a measurement of D-0-(D) over bar (0) mixing parameters in D-0 -> K-S(0)pi(+)pi(-) decays using a time-dependent Dalitz-plot analysis. We first assume CP conservation and subsequently allow for CP violation. The results are based on 540 fb(-1) of data accumulated with the Belle detector at the KEKB e(+)e(-) collider. Assuming negligible CP violation, we measure the mixing parameters x=(0.80 +/- 0.29(-0.07-0.14)(+0.09+0.10))% and y=(0.33 +/- 0.24(-0.12-0.08)(+0.08+0.06))%, where the errors are statistical, experimental systematic, and systematic due to the Dalitz decay model, respectively. Allowing for CP violation, we obtain the CP-violating parameters |q/p|=0.86(-0.29-0.03)(+0.30+0.06)+/- 0.08 and arg(q/p)=(-14(-18-3-4)(+16+5+2))degrees.