In order to reflect the risk of calcium stone formation, risk formulas have been described in the literature with the objective of being able to predict the further course of the stone disease. Some of these formulas are reviewed in this paper. Various results were obtained when different risk expressions were related to the severity of the stone disease. Although a reliable prediction of the future course of the disease most certainly cannot be made by analysis of the variables included in these expressions, several of the risk formulas differed significantly between patients with and without recurrent stone formation during a reasonable follow-up period. Some risk formulas might thus be helpful, at least to some extent, in selecting those patients in whom continuous stone formation can be anticipated and in whom active therapeutic measures should be beneficial and worthwhile. With an increased understanding of the mechanisms of calcium oxalate stone formation and our possibilities of measuring the relevant risk factors, it is likely that improved risk formulas with an increased predictive power can be developed. Until this becomes a reality, in most cases we have to combine important information on the history and clinical observations of the disease with a risk formula that offers a high degree of discrimination with respect to the risk of further stone formation.