Context • Anecdotal evidence concerning a relationship between human illnesses and a full moon is frequently claimed by as many as 81% of mental health workers. Previous scientific investigations have studied only the full-moon phase and its possible effect on psychiatric presentations. However, information is limited about all 4 phases of the lunar cycle and their effects on different types of psychiatric disorders. Objective • This study primarily intended to evaluate the number of psychiatric presentations to a hospital's emergency department across all 4 phases of the lunar cycle. The secondary objective was to investigate the statistical differences among 5 categories of common mental disorders in relation to the 4 lunar phases. Design • This study was an observational analytic cohort study. Setting • The study took place in the emergency department of a 140-bed, community-teaching hospital. Participants • Participants were 1857 patients who were aged >17 y and who had had a psychiatric component to a visit to the emergency department. Outcome Measures • Data from electronic medical records were collected for 41 consecutive months. The participants were divided into 5 diagnostic groups based on the Diagnostic and Statistical Manual of Mental Disorders, 5th ed (DSM-5). The study measured the number of psychiatric presentations for each group during the 4 National Aeronautics and Space Administration (NASA)-defined phases of the lunar cycle, and the study was statistically powered to detect small effects. Results • The following psychiatric presentations occurred: (1) 464 during the new moon; (2) 483 during the first quarter; (3) 449 during the full moon; and (4) 461 during the third quarter (4-group overall χ2, P = .89). Differences between the 5 diagnostic categories across the 4 lunar phases were not statistically significant (4-group overall χ2, P = .85 for the 5 diagnostic categories). Conclusions • Although many traditional and nontraditional providers believe in effects caused by the full moon based on casual observation or anecdotal evidence, this perception was not supported in the current study. Furthermore, no evidence demonstrated increased psychiatric presentations during the other 3 phases of the lunar cycle. The study found that the lunar cycle did not have an effect on the incidence of psychiatric presentations or on the DSM-5 categories. If lunar effects exist, they are probably small or infrequent, making them difficult to validate statistically. The current study's results, in concert with those of most other studies on the subject, provide evidence that should help dismiss misconceptions about the magnitude or frequency of lunar effects on psychiatric illnesses.