A 35-year-old Japanese woman for whom a previous health checkup showed normal blood pressure and urinalysis results without serological abnormalities developed nephrotic syndrome with severe hypertension at 15 gestational weeks. The renal biopsy performed at 17 weeks of gestation showed severe glomerular capillary endotheliosis. By means of electron microscopy, no electron-dense deposits were observed in glomeruli, and foot-process arrangement was normal. Histological findings indicated the patient's glomerular damage was caused by the mechanisms of preeclampsia. The patient underwent an elective abortion at 18 weeks of gestation. Clinical abnormalities vanished completely within 3 months after the elective abortion, which provided additional evidence that proteinuria and hypertension were caused purely by pregnancy. In general, the term preeclampsia refers to new onset of hypertension and proteinuria after 20 weeks of gestation. When proteinuria or hypertension is newly observed before 20 weeks of gestation, they are practically associated with triploidy, trophoblastic disease, or antiphospholipid syndrome. However, our case was not associated with them. Therefore, we called this case "pure" preeclampsia. We confirm the notion for the first time that preeclampsia associated with glomerular capillary endotheliosis can occur before 20 weeks of gestation. In addition, this report describes the earliest onset of preeclampsia compared with previously published reports. We also discuss causes of preeclampsia in early gestation and refer to the issue of the application of renal biopsies during pregnancy.