Here u go ...

Positive :
1. all possible single letters entry (EX: A,B etc)
2. All possible combination of multiple letters (EX: AB, 
ABC etc)
3. All possible duplicate letters (EX: AA, BB, AAA etc)

Negative:
1. Use integers
2. Use special chars etc

Positive Test Casee
1) All possible single Letters--System should display all 
entries that start with entered letter. Enter or selct by 
mouse should select the entry & display properly in list 
box.
2)All possible combination of multiple characters.--System 
shoul display all the entries that match the use iput.Enter 
or selct by mouse should select the entry & display 
properly in list box.
3) Check For Scroll Property of the combo box. If you enter 
A then list will display all entries that start with A use 
scroll bar or button on mose system should scroll within 
entries that start with A.
4)Use of Delete or Backspace should delete the entry from 
the list box. 

Negative Test cases

1) If compulsory check for blank
2) Try entering the value that is not in the list e.g. If 
list display Countries try entering Maharashtra etc. System 
should not allow user to enter value that is not in the 
list.
3)

use the decision table to find out all the possible
combinations.
A A B C D E ...........
B
C
D
E
.
.
.
You can get all the combinations also check the lenght of
the cities.

Nagative can be if you enter any blank, special characters,
numbers list should not display any cityname.