In order to identify possible aetiological factors in the genesis of physiological hydronephrosis in pregnancy, the degree of pelvic-calyceal dilatation in 90 asymptomatic pregnant women was correlated with levels of plasma oestradiol, progesterone, 24-hour urinary oestriol, the site of the placenta, birthweight of the fetus, and pelvic inlet measurements. A grading system based on maximum calyceal diameter was used; 90% of the patients were found to have at least mild dilatation on the right side. No correlation was demonstrated between the degree of hydronephrosis and the levels of oestradiol, progesterone and 24-hour urinary oestriol excretion. The birth-weight of the fetus and its relationship with the pelvic inlet measurements also did not correlate with the occurrence of hydronephrotic changes in the kidneys. The only significant positive finding was a higher incidence of moderate and severe hydronephrosis occurring in patients with a right-sided placenta than compared with the left (x2 = 4.77; p less than 0.05), although the sensitivity and specificity in predicting hydronephrosis from a right-sided placenta is low (53% and 66% respectively). Our results support the hypothesis of a mechanical aetiology in the genesis of pregnancy hydronephrosis, where vascular compression on the ureters may be an important contributory factor. Our study has also shown that urinary tract infection and reduction of creatinine clearance were not more common in patients with moderate or severe pelvic-calyceal dilatation.