What are the top causes of small bowel obstruction?

Upright abdominal film showing multiple air-fluid levels.

Most common causes of small bowel obstruction in adults:

#1.  Adhesions after abdominal surgery (60%)
#2.  Malignancy (20%)
#3.  Hernia (10%)
#4.  Inflammatory bowel disease (5%)

Intussusception is the most common cause of small bowel obstruction in children.

References:  
1.  Nobie, BA.  Small-Bowel Obstruction.  Emedicine.  Available at http://emedicine.medscape.com/article/774140-overview#aw2aab6b2b3.  Accessed March 3, 2012.