A recently announced change in policy by U.S. Citizenship and Immigration Services (USCIS) will force applicants for adjustment of status (a green card, when applied for within the U.S.) to be much more attentive to the timing of their medical exams. In short, USCIS will now treat the results of these exams (on Form I-693, completed by a physician) as good for only one year beyond the date the applicant submits them. You will have to make sure that no more than a year has gone by between having the exam done and submitting the results to USCIS.
A year sounds like a long time, but owing to USCIS delays, it's entirely possible for a year to go by without being called in for the interview at which one's file is reviewed and green card hopefully approved. In the past, USCIS ordinarily simply extended the validity of the exam results, as a matter of convenience. Under the new policy, however, USCIS plans to issue a "Request for Evidence" (RFE) asking for new exam results in cases where the one-year period has expired. (No need to send in a new exam on your own -- wait to hear from USCIS on this matter.)
But there's good news. In follow-up correspondence to its initial policy announcements, USCIS stated that it would accept adjustment of status applications that do NOT contain the medical exam report yet. Applicants can choose to submit it later by mail, or bring it to their interview.
The basic underlying concern is, of course, that the medical exam is meant to reveal whether the applicant has any health issues that make him or her inadmissible to the U.S. -- and health matters can change very quickly. See How Health Matters Can Make You Inadmissible to the U.S. for more on this.