Summary: 1. Body condition is a major concept in ecology addressed in countless studies, and a variety of non‐destructive methods are used to estimate the condition of individuals based on the relationship between body mass M and measures of length L. There is currently no consensus about the most appropriate condition index (CI) method, and various traditions have been established within subdisciplines in which ecologists tend to apply that method used previously by their peers. 2. Here, we present a reappraisal of six conventional CI methods: Fulton’s index (K = M/L³), Quételet’s index (BMI = M/L²), Relative condition (K_n, computed as the observed individual mass divided by the predicted mass M_i* = a L_i^b where a and b are determined by ordinary least squares (OLS) regression of M against L), Relative mass (W_r, where a and b above are determined from a reference population), the Residual index (R_i, the residuals from an OLS regression of M against L) and ancova. We compare the performance of these methods with that of the Scaled mass index, a novel method which was previously shown to perform better than R_i as a predictor of fat and other body components [J. Peig & A.J. Green (2009) Oikos, 118, 1883]. 3. To be reliable, a CI method must successfully account for the changing relationship between M and L as body size changes and growth occurs (i.e. for the scaling relationship between M and L). Using data from three species of small mammals we show that, unlike the Scaled mass index, all six conventional methods fail to do this, and as a result they consistently lead to significant differences in CIs between age classes and sex that are a mere consequence of changes in body size. The Scaled mass index was also particularly successful at detecting changes in CI resulting from high levels of contaminants.