Also, recommend the SAS "companion documentation" for your OS - available at the SAS support http://support.sas.com/ website.
Recommend a post-reply from the OP with a specific data example, ideally shown in a SAS generated log demonstrating the specific problem - suggest using the SAS output format BEST. to represent your variable.
Suggested Google advanced search argument, this topic / post:
sas numeric precision +"z/os" site:sas.com
Message was edited by: sbb
Also no context to this problem has been supplied, for example what is this 16-digit number used for? Also is it a whole number or are there decimals? This information is useful to providing the best advice.
For example, in banking most credit card numbers are 16 digits long (4 groups of 4 digits), but are held as character data to avoid any precision or formatting issues.