Inclusion Criteria:

• 1. Subjects must be diagnosed with PD according to conventional criteria.
• 2. Subjects must report having constipation and fulfill Rome III criteria19 for chronic constipation: at least 3 months, in the last 6 months with two or more of the following: i. Less than 3 SBM's per week ii. Straining with defecation more than 25% of the time iii. Lumpy or hard stools with defecation more than 25% of the time iv. Sensation of incomplete evacuation with defecation more than 25% of the time v. Sensation of anorectal obstruction or blockage with defecation more than 25% of the time vi. Use of manual maneuvers to facilitate defecation more than 25% of the time
• 3. Patients will be encouraged to use only lubiprostone for constipation. If they use any other agents they will need to record this use in their diary; any BM that occurs within 24 hours of the other agent used will be recorded, but not be counted as a SBM.
• 4. Patients or patients' caretaker(s)/ legal guardian must be able to read, understand, and accurately record data into the diary to guarantee full participation in the study.
• 5. Patients over the age of 50 must have had a colonoscopy or sigmoidoscopy within 5 years.
• 6. Patients or patient's caretaker(s)/legal guardian must be willing and able to provide informed consent before beginning the study.

Exclusion Criteria:

• Evidence of structural abnormality of the gastrointestinal tract or diseases/conditions that affect bowel transit including gastric, small bowel or colonic resection (appendectomy, cholecystectomy, benign polypectomy are allowed); history of colon cancer, history of inflammatory bowel disease (Crohn's disease or ulcerative colitis); insulin-dependent diabetes mellitus, history of Hirschsprung's disease, progressive systemic sclerosis (scleroderma), anorexia nervosa; other diseases or conditions that in the opinion of the investigator significantly affect bowel transit. Subjects with constipation secondary to any other documented cause.
• Planned use of drugs or agents during pretreatment phase onward that affect gastrointestinal motility and/ or prescription including laxatives including stool softeners (patients experiencing significant constipation may use a laxative as rescue medication if needed); antidiarrheals (in case of significant diarrhea loperamide may be used if needed); antacids containing magnesium or aluminum salts (only calcium containing ones are allowed); anticholinergics, antispasmodic agents (e.g., Librax, Donnatal, dicyclomine); erythromycin and other macrolides; octreotide; ondansetron or other 5-HT3 antagonists; opioids/narcotic analgesics; prokinetics (metoclopramide); serotonin re-uptake inhibitors or tricyclic antidepressants (allowed if constant doses for at least 1 month before treatment); calcium antagonists (allowed if constant doses for at least 1 month before treatment).
• Subjects with any significant cardiovascular, liver, lung, renal, psychiatric or neurological diseases (not including PD).
• Patients with previous allergic reaction or lack of tolerability to lubiprostone.
• Current or recent history (within 12 months) of drug or alcohol abuse.
• Pregnancy or breast feeding.
• Fertile women (defined as those who are not surgically sterile, are not >1 year post-menopausal or who are not currently using or complying with a medically approved method of contraception). Lubiprostone has not been studied in pregnant women and should only be used during a pregnancy if the potential benefits justify the potential risk to the fetus. Women should have a negative pregnancy test before beginning treatment with lubiprostone and need to practice effective contraceptive measures