The purpose of the study was to assess the influence of transmission error on primary care prescribing after secondary care discharge and out-patient consultation. For all secondary care in-patient discharges and out-patient letters over 4 weeks, the quality and timeliness of information sent to general practitioners were monitored. Discrepancies between discharge information and practice records were also noted. The sources of error were classified and quantified. Secondary care discharge and out-patient information sent to six general medical practices in Calderdale and Kirklees Health Authority, England. Information required for appropriate prescribing was missing from 11% of discharge advice notes and 15.2% of discharge letters. Discharge advice note and discharge letter were both available when needed in 8.1% of cases. Comparison between discharge advice notes and discharge letters showed discrepancies in one-third of cases. Important information was missing from 25% of outpatient letters. 96.1% of discharge advice notes arrived within 8 days of discharge. Patient requests for medication were rarely made before the necessary information arrived. Outpatients were provided with 28 days supply of medication in 92.7% cases, and 91.8% of outpatient letters arrived at the practice within 21 days. Transcription errors were common at all stages including transfer of secondary care information to primary care notes. To prevent potentially harmful drug errors further effort is needed to improve systems for information transfer between secondary and primary care. Transcription errors that arise within primary care have been previously unreported. The introduction of electronic prescribing and communication offer a technical solution to significant problems. However, the success of future solutions should not be assumed. (12 refs.)