Establishing effective communication between general practitioners (GPs) and medical specialists is a key component of the referral system. Written communication between GPs and medical specialists is the most common communication tool. This study was conducted to evaluate quality (information content) of the referral letters written by GPs and addressed to gynecologists and cardiologists. We evaluated quality of the referral letters through a cross-sectional study in the villages of Sarab city, located in East Azerbaijan Province, Northwest Iran. The study was conducted during August and September 2015 in which a total of 400 referral letters were evaluated according to specific quality criteria. Cluster sampling was implemented and data were collected using an instrument designed by the Department of Family Medicine at the University of Manitoba, Canada. A specifically designed referral form was used to refer pregnant women to gynecologists. Referrals addressed to gynecologists showed better quality in comparison to cases referred to cardiologists. Legibility of referral letters was 73%. It is recommended that agreed-upon referral letters be designed cooperatively for different groups of diseases. Furthermore, primary health care providers should be trained to write proper referral letters.