This is a double-blind, randomized, parallel group, dose ranging study of subcutaneous methylnaltrexone to be conducted in patients with advanced medical illness and poorly controlled opioid induced constipation. Patients will be randomized to one of three fixed dose levels of SC MNTX.

Inclusion Criteria:

1. Must have advanced medical illness (cancer, HIV, etc) and be receiving palliative care
2. Must be on opioid medication for at least 2 weeks with no expectation of significant change in regimen
3. Must have constipation
4. Must be 18 yrs or older

Exclusion Criteria:

1. Concurrent use of medications other than opioids which might interfere with gastrointestinal motility
2. Patients who received any experimental drug in the last 30 days
3. Patients with active peritoneal cancer (ovarian, etc.)
4. Patients with active diverticulitis or diverticulosis