The introduction of programmable as well as non-linear hearing instruments has shown that audiogram-based fitting procedures are inappropriate for meeting the individual needs of hearing aid users with sensorineural loss. Particularly the present and future development of hearing instruments with advanced analog and digital signal processing requires suitable fitting procedures. In this context, we developed an adaptive fitting strategy (ScalAdapt) using category loudness scaling. In this paper the procedure is described and tested using programmable 3-channel AGC instruments. However, it can be modified for any other hearing aid design. The underlying idea is to normalize the aided loudness perception for two input levels, one around the most comfortable level (MCL) and the other just below the uncomfortable level (UCL), as anchor points in each channel by interactive fine-tuning of the fitting parameters (channel gain, compression onset, compression ratio, etc.). The benefits of ScalAdapt have been evaluated by a field test study in 17 experienced hearing aid users. The results showed that ScalAdapt provides consistently higher objective (speech recognition in noise) and subjective benefits (self-assessment inventory) than for instance NAL-based fittings.