The relationship between blood pressure and the 24-hour urinary excretion of four cations (Ca, Mg, K, Na) was investigated in a random sample of 688 inhabitants of two Belgian towns. In 160 youths aged 10–19 years, systolic/ diastolic blood pressure averaged 118 ± 12.6/65 ± 8.6 mmHg (mean ± standard deviation) and the urinary excretion of the four urinary cations was broadly similar in both sexes. Adjusting for body weight removed the strong relationship between blood pressure and age, but a positive relationship between systolic pressure and pulse rate emerged. The only association between blood pressure and a urinary constituent was with calcium excretion, and this correlation was no longer apparent after adjusting for weight. In 528 adults aged ≥20 years, systolic/diastolic pressure averaged 130 ± 14.4/77 ± 9.8 mmHg and the urinary excretion of the four urinary cations was significantly ( p < 0.001) higher in male than in female subjects. In these adults, both systolic and diastolic pressure were strongly and independently correlated with age and body weight. Systolic pressure in women was also significantly and positively related to pulse rate ( r = +0.20; p < 0.001). After adjusting for age and body weight, systolic and diastolic pressure in men were significantly and negatively correlated ( p < 0.001 and p < 0.01, respectively) with urinary potassium excretion. Diastolic pressure in men was weakly but positively correlated with calcium excretion ( p < 0.05 after adjusting for body weight, age and urinary potassium excretion). The present study indicates that urinary potassium is a consistent and negative predictor of both systolic and diastolic pressure in adult men, whose diastolic pressure was also weakly and positively associated with urinary calcium. In youths and female subjects, the single 24-hour urinary excretion of the four cations did not contribute to the prediction of blood pressure.