Some patients are less lucky than others and have several diseases at the same time. When this occurs the diseases can be seen as completely separate, or as a reflection of a deeper common aetiology or core pathogenic process. However, keeping in mind Adams's statement that "co-occurrence does not imply association" , we have attempted in this chapter to cover all the diseases associated with hidradenitis suppurativa (HS) except for Crohn's disease. It is our hope that by exploring the described associations it will be possible to suggest avenues of future research into the aetiology and pathogenesis of HS. The observations can lead to a segregation of the readership into aggregators and dividers. Aggregators tend to aggregate similar diagnoses into one, whereas dividers split closely related diseases into subgroups. Either approach may be appropriate, providing it is argued from a rational perspective; the argument cannot only be theoretical or practical, but must take the clinical reality into consideration in all its complexity. We have first described cutaneous diseases including follicular occlusion diseases and pigmentation disorders of the skin folds (Dowling-Degos and Kitamura's diseases), then rheumatologic diseases, neutrophilic diseases and finally cancers (cutaneous and others). By describing the plethora of diseases in which a possible causal association has been contemplated, it is our hope to stimulate the creative curiosity.