At the Radiation Effects Research Foundation, medical examinations have been conducted biennially since 1958 on a fixed population of approximately 20,000 individuals. Blood pressure measurements and electrocardiographic (ECG) recordings are available for 6,569 individuals who were monitored for at least 11 of the 13 2 year intervals between 1958 and 1984. Data from 601 individuals who had satisfied the Foundation's ECG diagnostic criteria of left ventricular hypertrophy ("Kagan-Yano code") on at least one occasion were reviewed. Both the development and the regression of ECG left ventricular hypertrophy were ascertained in 61 subjects (17 men and 44 women). During the course of development of ECG left ventricular hypertrophy, hypertension (including borderline cases) was noted in 83.3% of the subjects. The most common pattern of ECG left ventricular hypertrophy development was high voltage, followed by ST-T changes. In about half of these cases, the condition of hypertrophy regression was associated with lowering of blood pressure, marked by the disappearance of high voltage ECG readings.