Contributor: SCOTT STONE Function CheckPrime(a : integer) : boolean; Var x : integer; y : integer; Begin y:=0; for x:=1 to (a div 2) do {Only #s up to half of a can be factors} begin if (a mod x)=0 then y:=(y+1) end; if y=2 then checkprime:=true else checkprime:=false; if a=1 then checkprime:=true; End; You see, only prime numbers have exactly two factors, themselves and one. With the exception of One. Therefore you have a specific IF for the number one. One is prime, yet its only factor is one. I think - Is one prime or not? Anyway, remove that line if it isn't, the function will work.