Objectives: To determine rates of pregnancy smoking concealment compared to behavioral observation and to identify factors predicting untruthful denial. Methods: Review of 843 delivery charts. Results: Based on observation during delivery hospitalization, 8% of smokers denied at hospital admission, 16% denied throughout prenatal care. Compared with those admitting smoking, false deniers had higher levels of education, incomes and adequate prenatal care utilization; and were less likely to have drug use, STDs, or hepatitis C. Observation was a valid tool for assessing smoking status. Conclusions: Typical concealers were those considered lower risk, increasing the chance they would go undetected as smokers.