Spontaneous quitters are prepregnancy smokers who quit by the time of their first antenatal visit. We recruited 192 self-declared spontaneous quitters and 407 smokers at their first visit to the antenatal clinic at the Royal Women's Hospital during April, 1994-May, 1995. Spontaneous quitters made up 23% of prepregnancy smokers. Information about self-declared quitters and smokers was collected by self-completed questionnaires. Urine samples collected at the first visit and in late pregnancy were assayed for cotinine to validate smoking status. A cut-off urinary concentration of > or = 653 nmol/L cotinine was used to determine active smoking. At the first visit, 20% of the self-declared spontaneous quitters were smoking and by late pregnancy, regardless of their initial biochemically verified status, 27% were smoking. Spontaneous quitters were different from women who said they were still smoking at their first antenatal visit, in a range of demographic variables and measures of addictive behaviour.