A study was conducted to determine the risk of upper respiratory disease among deployed U.S. Navy shipboard personnel. Between January and June 1989, a Patient Encounter Form was used to collect disease information from 10 U.S. Navy ships. Patients with a diagnosis of upper respiratory infection (URI) on initial sick-call visit were used in this study. Information on time spent in various ports and time at sea for each of the 10 ships was also collected. A classification scheme was developed to permit daily estimates of URI rates and to indicate whether the ship was in port or at sea. Of 967 cases of URI, 64.4% occurred while at sea, with an average daily rate of 0.5/1,000 crew members; 35.4% of the cases occurred while in port, with an average daily rate of 0.4/1,000 crew members. There was an increase in URI rates after 9 days at sea or in port. These data suggest that there are defined periods of increased transmission of upper respiratory infections aboard ships, both at sea and in port.