[영문]BACKGROUND:: The cesarean rate has served an integral role in the monitoring of obstetrical care, and in 2002, the national rate reached 26.1%, the highest ever reported. OBJECTIVE:: We sought to describe the effect of clinical complications on hospital cesarean rates. RESEARCH DESIGN:: This was a population-based cohort study. SUBJECTS:: All laboring women without a previous cesarean who delivered in California in 1995 as reported through public-use hospital discharge data were included. MEASURES:: Women with and without maternal, fetal, or placental complications were compared with respect to cesarean use. Using recursive partitioning algorithms, women with complications were stratified into clinically homogeneous categories, which were analyzed separately with respect to cesarean use. RESULTS:: The 443,532 women delivered at 288 hospitals and included 116,170 women (26.2%) in the complicated group (cesarean rate 22.6%); and 327,362 women (73.8%) in the uncomplicated group (cesarean rate 6.7%). At the hospital level, the cesarean rates among the complicated and uncomplicated patients respectively were: median 23.5% (range, 2.2?9.9%); and median 6.5% (range, 1.8?18.2%). Recursive partitioning algorithms suggested 16 distinct clinical categories, with cesarean rates varying from 8.9% for women with asthma to 84.5% for women with an unengaged fetal head. CONCLUSIONS:: Cesarean rates varied widely across complication types, and complication-specific rates varied widely among hos-pitals. Although the presence of pregnancy complications upon hospital admission comprised the strongest factor affecting first-time cesarean use among laboring women, the importance and interdependence of these clinical conditions has yet to be incorporated into commonly used models for cesarean rate comparisons.