Obstructive
(1) prostatic enlargement (benign, cancer)
(2) bladder stones, neoplasm
(3) urethral stricture, stones, foreign bodies
(4) uterine or gastrointestinal mass (cancer, fecal impaction) compresses bladder neck
(5) pelvic organ prolapse: cystocele or rectocele
(6) phimosis, paraphimosis
(7) gravid uterus
Infectious / Inflammatory
(8) prostatitis
(9) urethritis
(10) vulvovaginitis
(11) herpes simplex virus (Elsberg syndrome)
Pharmacologic
(12) anticholinergics
(13) hormonal agents: estrogen, progesterone, testosterone
(14) sympathomimetics: phenylephrine (neosynephrine), pseudoephedrine (Sudafed)
(15) nonsteroidal antiinflammatory drugs via inhibition of prostaglandin mediated destrusor muscle contraction
(16) opioid analgesics
Neurologic
(17) brain lesion: stroke, multiple sclerosis, concussion, tumor, normal pressure hydrocephalus, Parkinson's disease
(18) peripheral neuropathy: diabetes, Guillain-Barre, herpes zoster, lyme, tabes dorsalis
(19) spinal cord lesion: disk herniation, trauma, tumors, spinal stenosis, meningomyelocele, transverse myelitis
Source
Selius, B. and Subedi, R. "Urinary Retention in Adults: Diagnosis and Initial Management" American Family Physician. 1 March 2008.