Informal complaints on health services: hidden patterns, hidden potentials. 
To examine the prevalence of informal complaints on health services among clients of Health Maintenance Organizations, and explore demographic correlates.  Such complaints are a potentially important source of information regarding quality of healthcare.  Primary data were collected by a phone survey from a nationwide random sample of 1500 persons aged 21+ in Israel.  About 25% of the respondents reported a cause to complain, but only 9.5% actually complained.  About 75% of the complainants submitted their grievances informally at the local level.  Only a minority (17%) appealed to official bodies established by law.  Minority groups and recent immigrants had significantly lower rates of reasons to complain and actual complaints.  Suggestions are made for outreach efforts to socially vulnerable groups and for developing organizational mechanisms for capturing and using future complaints submitted informally to front-line employees, which are the bulk of the complaints.  Further research is needed regarding factors affecting customers complaining and non-complaining behavior, including factors that specifically affect the behavior of minority groups.