The National Practitioner Data Bank became operational September 1, 1990, as a flagging system to identify health care practitioners who may have been involved in incidents of medical incompetence. Query volumes have grown substantially over the Data Bank's first 4 years of operation. The greatest increase has come in the number of voluntary queries. By the end of 1994, the Data Bank had processed more than 4.5 million requests for information on practitioners, more than 1.5 million of which were received in 1994 alone. The proportion of queries for which the Data Bank contains information on the practitioner in question has grown as the Data Bank has come to contain more reports. During 1994, 7.9 percent of queries were matched. The Data Bank contained more than 97,500 reports at the end of 1994. More than 82 percent of the reports concerned malpractice payments. Licensure reports made up the bulk of the rest. Physicians predominate in reports, accounting for slightly more than 76 percent of the total. The remainder are related to dentists (16 percent) and all other types of practitioners (8 percent). Since reporting of adverse actions is mandatory only for physicians and dentists, the proportion of reports attributable to these types of practitioners is higher than it would be if adverse action reporting requirements were uniform for all practitioners. State malpractice payment rates and adverse action rates vary widely, but a State's rate in any given year is highly correlated with its rate in any other year. State malpractice rates are not strongly correlated with adverse action rates, neither are the rates for physicians strongly correlated with those for dentists. There is a weak tendency for States with smaller physician populations to have higher levels of licensure and privileging actions.