Field-portable detectors have been frequently used in routine monitoring and hazard assessment studies. However, there have been few thorough attempts to evaluate their potential as an alternative to the traditional procedure of collecting samples and analysing them in the laboratory. Thus, in this study the two approaches were compared in terms of their utility for monitoring 137Cs activity in the Nyänget catchment in northern Sweden. The objectives were: (i) to determine the 137Cs activity in soils associated with three types of vegetation, (ii) to map the geographical distribution of 137Cs using the portable NaI detector connected to a GPS system (GDM-40), (iii) to identify 137Cs anomalies in the catchment, and (iv) to compare the measurements obtained with the NaI detector and traditional sampling followed by laboratory analysis. Our results demonstrate that the GDM-40 has very good potential for making 137Cs inventories and for detecting 137Cs anomalies within large areas. The GDM-40 measurements identified differences between different hydrological areas that were not determined with the soil sampling method. The GDM-40 method is much faster than a traditional soil sampling method. However, soil sampling cannot be totally excluded because it is needed to calibrate the GDM-40. The agreement between the 137Cs activity values obtained by the two approaches was 20% which is good in the field where so many factors vary.