The diagnosis of acne is usually easy, but there are some pitfalls to be avoided. ‘Keloid acne of the neck’ and beard folliculitis are not acnes in the usual sense: both are inflammatory and fibrous reactions of the hair follicles and frizzy hair; no retentional lesions, blackheads and microcysts – are visible. Gram negative folliculitis classically occurs in acneic male subjects who have undergone extensive treatment with general antibiotics or local antiseptics, but ‘de novo’ cases do exist. On black skin, this condition is not exceptional, it occurs in both sexes and usually takes the nodular form. The diagnosis should be considered if there is any aggravation of acne which is resistant to classic treatment, with painful nodules on the cheeks. Treatment is based on appropriate antibiotherapy for several weeks and possibly, in a second phase, on Isotretinoin. Pityrosporum folliculitis occurs mainly on the trunk. More frequent in men than in women, it is chiefly observed in subjects living in a hot, humid climate. Demodicidosis is manifested by outbreaks of papular or papulopustular lesions of the face. On black skin the principal differential diagnosis is acne. The presence of numerous parasites is necessary for diagnosis. Clinically speaking, an important sign is when the eyelids are affected. Ivermectin is effective. Acneiform dermatitis may be induced by depigmenting preparations containing powerful dermocorticoids. It is therefore important, in cases of very inflammatory acne, to look for the other clinical signs of voluntary depigmentation. In countries where it is endemic, lepromatous leprosy should be considered. Other common dermatitis may simulate acne or else be associated with it, such as eruptive hidradenoma or molluscum contagiosum. Analysis of the different elementary lesions and the absence of retentional lesions generally enable a diagnosis to be established.