The purpose of this study was to investigate the relationship between swimming economy, energy cost to move the body per unit distance (CS) at a given velocity (v) and the potential determinants, i.e. performance level, body size, swimming technique and v. A total of 101 males were studied. Three performance levels (A, B, C) were determined, ranging from the slower (A) to the faster times (B, C). At level C and at 1.1 m.s-1, CS 1.1, was reduced by 55% and 25% when compared with levels A and B and when calculated per unit of surface area (SA) and unit of hydrostatic lift (HL). For the whole group of swimmers, CS 1.1 = 21.88 SA-2.15 HL + 5.9 (r = 0.56, P less than 0.01). Among the 101 swimmers, three other groups were selected to evaluate specifically the influence of arm length and swimming technique on CS, i.e. arm or leg swimmers and sprinters versus long-distance swimmers, CS was significantly (P less than 0.05) lower for long-arm swimmers, arm and long-distance swimmers than for short-arm, leg and sprint swimmers by 12%, SD 3.3%, 15%, SD 3.8% and 16.5%, SD 3%, respectively. For all groups, CS increased with v on average by 10% every 0.1 m.s-1. It is concluded that technical ability cannot be interpreted directly from CS. Performance levels, body size, swimming technique and v at which the measurements are obtained must be also taken into account.