A method fits a binaural hearing aid system for a person with a small or moderate asymmetrical hearing loss. The method comprises providing first hearing loss data for a right ear of a user; providing second hearing loss data for a left ear of a user; determining a difference between the first and second hearing loss data; classifying the degree of similarity of the first and second hearing loss data; determining basic hearing loss data to for each of the first and second hearing instruments depending on hearing loss classes; and calculating frequency dependent target gain values for each of the first and second hearing instruments.