This study is being conducted to identify how much and how often pralatrexate, given with vitamin B12 and folic acid, can be given safely to patients with cutaneous T-cell lymphoma (CTCL) that has relapsed (returned after responding to previous treatment) or is refractory (has not responded to previous treatment). It is also being conducted to get information on whether or not pralatrexate is effective in treating relapsed or refractory CTCL.