The present work describes a detailed comparison between SOHO-CDS observations of active region loops with a static, isobaric loop model developed assuming a temperature-independent heating function in the energy balance equation and a variable loop cross-section. The loop model is described in Landini & Landi (2002). Observations of an active region recorded by CDS have been analyzed. Additional data from the EIT and MDI instruments on board the SOHO satellite, and broad band soft X-rays images from the Yohkoh satellite, have been used to complement the CDS dataset. CDS monochromatic images from lines at different temperatures have been co-aligned with EIT, MDI and Yohkoh images and a loop structure has been identified. Two other loop structures are visible but their footpoints are not clearly identified, and have not been analyzed. Electron density, temperature and pressure along the selected loop structure have been measured by means of line ratio techniques. These quantities have been used to test the assumption of constant pressure adopted in the theoretical model, and to compare their values with its predictions. The loop filling factor has also been estimated from the CDS data after assumptions on the loop geometry have been made. Comparison with CDS data has shown that a classical model is not able to reproduce the observations; despite the large uncertainties, mainly given by the limited CDS spatial resolution, indications suggest that agreement occurs only if an ``ad hoc'' isothermal region is added on top of the loop and a large conductive flux at the base is assumed. Suggestions for improvements of theoretical loop models and further studies with the EIS instrument on Solar-B, due for launch in 2005, are given.