Our aim was to clarify the perinatal outcomes of and risk factors for hypertension that is first detected after labor onset (labor onset hypertension, LOH), which may be a risk factor for eclampsia and stroke during labor. A total of 1349 parturient women who did not exhibit preeclampsia or gestational hypertension prior to labor were examined. The patients were classified into four groups: the normotensive (n=1023) (whose systolic blood pressure (SBP) remained below 140 mm Hg throughout labor), mild LOH (n=241) (whose maximum SBP during labor ranged from 140 to 159 mm Hg), severe LOH (n=66) (whose maximum SBP during labor ranged from 160 to 179 mm Hg) and emergent LOH groups (n=19) (whose maximum SBP during labor was greater than 180 mm Hg). The perinatal outcomes and patient characteristics of the four groups were compared. Twenty-four percent of the pregnant women who remained normotensive throughout pregnancy developed hypertension during labor. One of the patients in the emergent LOH group developed eclampsia. The blood pressure at delivery and frequencies of hypotensor use, interventional delivery and low Apgar scores differed significantly among the four groups. The following risk factors for severe/emergent LOH were extracted: being over 35 years old, a body mass index at delivery of >30, an SBP at 36 weeks' gestation of 130-134 mm Hg, an SBP at admission of 130-139 mm Hg, proteinuria (a score of 2+ on the dipstick test) and severe edema. The risk factors for severe/emergent LOH were identified in this study. In high risk cases, repeatedly measuring maternal blood pressure during delivery might help detect critical hypertension early.