The solubilization of the practically water insoluble drug furosemide by guest:host inclusion complexation with 4-sulphonic calix[n]arenes has been reported. The 4-sulphonic calix[n]arenes are water-soluble phenolic cyclooligomers that form inclusion complexes with neutral molecules. The solubility of furosemide in acidic (pH < 4) aqueous solutions containing increasing concentrations of the calixarenes was determined at 30°C and the concentration of furosemide in solution was determined by HPLC. Results showed that the molecular size of the 4-sulphonic calix[n]arenes and the concentration of the calix[n]arenes significantly influenced the increase in the solubility of furosemide. 4-Sulphonic calix[6]arene improved the solubility of furosemide the most (± 104%) followed by 4-sulphonic calix[8]-arene (±84–102%), while 4-sulphonic calix[4]arene increased the solubility of furosemide the least (±73–81%). The increase in furosemide solubility afforded by the calixarenes was most probably the result of the incorporation of the non-polar portions of the furosemide molecule into the non-polar cavities of the calixarenes similar to furosemide:cyclodextrin complexes. The driving force for this interaction was the reduction in the non-polar-water interfacial surface area when the furosemide (guest) molecules were inserted into the 4-sulphonic calix[n]arenes (host).