When an interlabial mass is seen on physical examination in a little girl, there is often confusion about its etiology, its implications, and what should be done next. Five common interlabial masses, which superficially are strikingly similar, include a prolapsed ectopic ureterocele, a prolapsed urethra, a paraurethral cyst, hydro-(metro)colpos, and rhabdomyo sarcoma of the vagina (botryoid sarcoma). A prolapsed ectopic ureterocele occurs in white girls as a smooth mass which protrudes from the urethral meatus so that urine exits circumferentially. A prolapsed urethra occurs in black girls and resembles a donut with the urethral meatus in the center. A paraurethral cyst is smaller and displaces the meatus, so that the urinary stream is eccentric. Hydro(metro)colpos from hymenal imperforation presents as a smooth mass that fills the vaginal introitus, as opposed to the introital grapelike cluster of masses of botryoid sarcoma. Recommendations for efficient imaging are presented.