Interpretation of observational studies is difficult, particularly in cross-sectional studies, because the direction of cause and effect may be difficult to assess Did the “outcome” affect the measured exposure level, or did the exposure affect the outcome? In this paper, the authors describe a pattern, the “checkmark pattern,” which can arise in cross-sectional studies. This pattern is characterized by higher levels of the outcome in an unexposed comparison group than in some subgroups of the exposed. The pattern, if seen in certain types of observational studies, suggests that the “outcome” variable may have affected the measured exposure level. Recognition of the pattern may help the epidemiologist to decipher the causal sequence Two examples illustrate the issues involved. Am J Epidemiol 1992;135:926–35.