Compulsive counting is a common symptom of obsessive-compulsive disorder. People with counting compulsions may count because they feel that certain numbers have a special significance, and therefore specific actions must be performed a certain number of times. For example many people with OCD feel that the number four is especially significant, and will therefore do things in sets of fours. For example if they smoke one cigarette, they may feel a sense of incompleteness unless they smoke three more. So they will count the number of cigarettes to be sure they have met the numerical goal.

People with counting compulsions may also count without thinking about a specific number. They might count their steps when walking, count tiles on the ceiling, or count cars driving past. The counting may be mental or aloud.

People with counting obsessions do not always have a reason for counting. Even though someone who has contamination fears often knows their washing concerns are overboard, they  usually feel there is a chance the fear is somewhat based on reality. With numbers, the OCD does not always activate because it is worried about a certain outcome. Often taking 4 steps within say one block on the sidewalk, or looking away from a clock at a certain number may just feel right, rather than be driven by fear. Often people with OCD will have a primary subtype, i.e. sexual obsessions, but will have the numbers as more of a secondary problem. In cases like this, it is not as stressful or intrusive.

Even though OCD is often illogical, many fears seem very logical in the mind of the sufferer, with seemingly very clear events leading straight to why a fear is valid and true. Unless very severe, adults with numerical obsessions usually realize there is no basis to the number fears (unless perhaps driven by religious fears over evil numbers, e.g. 666). Nonetheless, number fears can enter the mind and say "look away from the clock at 12:11 or your family will die of cancer." Maybe the mind said 12:11 because when you add the numbers it comes out to 4 and just feels right. In a case like this, the OCD sufferer will say to themselves, "that is ridiculous," but then the OCD will say “yes, but just in case look away at this time.” If the person looks away at a bad time, the OCD may pester to keep looking until an acceptable number is found. The person may even have to make a choice. The OCD can say, look away at 12:56 and your family will die of cancer, but look away at 1:07 and you will go blind. One may then choose the 1:07 time just in case, to protect ones family, but will still be dealing with the fear of going blind. In this case they might take the blind sacrifice for their family, or spend a lot of time looking until a suitable look away number can be found.

Although a person with OCD may realize this is just the OCD, they continue to count or heed specific special numbers "just in case" and why tempt karma. These cases add up greatly with OCD, and reinforcing the checking just makes the OCD stronger. Which is why a sufferer may eventually become severe and consumed with counting rituals all day. If numbers are secondary to other OCD concerns, these fears atop of a brain that already is overtaxed can be the final straw.