A 22-year-old man with a long history of cannabis-use presented with a full blown mania. According to recent population-based studies, someone who starts using cannabis early in life runs the risk of developing not only psychosis but also manic symptoms and bipolar disorder. Further literature research showed that the use of cannabis influences both the onset and the course of bipolar disorder. When a patient presents with mania it might be advisable for the psychiatrist to take cannabis-use into account when making a diagnosis and prescribing treatment.