A family comprising 46 members of 4 generations is described; 21 members suffered from incomplete diabetes insipidus (DI) of central origin. The pedigree showed a dominantly transmitted condition. The onset is gradual and starts in early infancy. The clinical symptoms are highly variable and decline in the sixth decade. Plasma vasopressin (AVP) during water deprivation was significantly lower in the DI group than in the controls (4.2 +/- 0.5 vs. 10.6 +/- 1.7 ng/l) (p less than 0.01), the difference being more pronounced in the high osmolality range (4.8 +/- 0.7 vs. 14.4 +/- 3.1 ng/l) (p less than 0.01). Urine osmolality was lower (241 +/- 36 vs. 928 +/- 46 mOsm/kg H2O) (p less than 0.01) despite higher serum osmolality during water deprivation, rendering the ratio between urine and serum osmolality less than unity compared with greater than 3:1 in the control group (p less than 0.001). In two affected females, addition of a non-osmotic stimulus caused no increase in plasma AVP. The findings are consistent with a partial defect in the production or release of AVP and not with a dysfunction of the intracranial osmoreceptors. The variable features of incomplete DI indicate that to define the condition by excessive urinary output alone is insufficient. The ratio between urine and serum osmolalities after an appropriate osmotic stimulus together with plasma AVP measurements may be necessary to confirm the diagnosis.