Obesity and osteoarthritis (OA) co-exist in an increasing part of the population. The two diseases intertwine in several ways. The evolution in the population shows a tendency towards deterioration of both by increasing general age and weight. The two diseases share pathogenetic features and the development of one disease increases the risk of the other and may be the onset of a vicious circle. There is a link between treatments of these two diseases as well. There is now solid (gold) evidence that by treating effectively the obesity of patients with co-occurring OA, the functional status is dramatically ameliorated; the short-term results are equal to that of a joint replacement. The long-term efficacy of a weight loss remains to be shown. OA is definitely one of many diseases in which obesity must be taken seriously into account when planning a correct treatment of patients. The regimens used in the controlled studies of such patients are reviewed and it may be concluded that practical aspects of the dietary intervention in obese patients with OA are not different from general recommendations.