1 Introduction.- 1.1 Objectives.- 1.2 Reasons for Study of Inaccuracy of Smoking Habits.- 1.3 Epidemiological Evidence of an Association of Passive Smoking and Lung Cancer.- 1.4 Possible Explanations for the Association.- 1.5 First Attempts to Estimate Misclassification Bias.- 1.6 Difference in Conclusions and Need for More Data.- 1.7 Misclassification Not the Only Issue.- 1.8 Structure of the Monograph.- 2 General Considerations.- 2.1 Major and Minor Sources of Misclassification Bias in Passive Smoking Studies.- 2.2 Types of Information on Misclassification of Smoking Habit.- 2.3 Factors Which Might Affect Reported Smoking Habits.- 2.4 Use of Objective Markers.- 3 Evidence on Misclassification.- 3.1 Studies Using Carbon Monoxide and Thioeyanate as Objective Markers.- 3.2 Studies Using Nicotine or Cotinine as Objective Markers.- 3.3 Studies Involving Multiple Reports Concerning the Same Individual — Subject a Lung Cancer Case.- 3.4 Studies Involving Multiple Reports Concerning the Same Individual — Subject not a Lung Cancer Case.- 3.5 Other Evidence on Misclassification of Smoking Habits.- 4 Overview and Unanswered Questions.- References.- Appendix A Studies of Passive Smoking and Lung Cancer — A Brief Review with Special Reference to the Way the Smoking Habit was Obtained.- Appendix B Passive Smoking and Lung Cancer Association: A Result of Bias?.- Appendix C Major and Minor Sources of Misclassification Bias in Passive Smoking Studies — Mathematical Details.- Appendix D Trends in the Percentages of UK Men and Women who Have Never Smoked.