Why do some women have difficult labor whereas other women have relatively easy labor? A study of possible predisposing factors was conducted in 398 and 383 primiparas admitted for uncomplicated spontaneous labor at two hospitals. Psychosocial factors, notably anxiety about childbirth, and physical factors, especially maternal height and body mass index, were measured before the onset of labor. Difficult labor was defined in primiparas admitted to the hospital for spontaneous uncomplicated labor as (1) prolonged labor that was longer than 15 hours from admission until full dilation with vaginal delivery or (2) cesarean section. Anxiety about childbirth as self-reported before the onset of labor did not predispose to difficult labor at either hospital. Short height and heavy body mass index predisposed to difficult labor. Further work is needed to elucidate psychosocial factors predisposing to difficult labor.