A method for diagnosing autism in a subject is performed by determining the presence and concentration of specific peptides in the body fluid. The peptides have the amino acid sequences SKITHRIHWESASLL (SEQ ID NO: 1), SSKITHRIHWESASLL (SEQ ID NO: 2), and SSKITHRIHWESASLLR (SEQ ID NO: 3) with the molecular masses 1779 +/−1 Da, 1865 +/−1 Da and 2022 +/−1 Da, respectively.