1. The complement 3 (C3) phenotype distribution in 112 patients with essential hypertension was compared with the distributtion in 316 normotensive control subjects.
2. A significant increased frequency of the C3F-gene was found among the patients (0.2637 vs 0.1721, P = 0.0031), indicating an association between the C3F-gene and essential hypertension.
3. The relative risk of essential hypertension was calculated to be 1.90 for C3F-positive individuals.
4. The association between the C3F-gene and essential hypertension was stronger among the untreated patients, where a relative risk of 3.89 was found for C3F-positive subjects.
5. A significant negative correlation was found between the C3F-positivity and the severity of the hypertensive disease estimated by eye ground changes. This might be in accordance with a negative natural selection of C3F-positive hypertensive patients.
6. The study supports the hypothesis that immunogenetic factors may be of pathogenetic importance in essential hypertension.