In N-glycosylated glycoproteins, carbohydrate is attached to Asn in the sequence Asn-X-Ser/Thr, where X denotes any amino acid. However, the presence of this consensus peptide does not always lead to glycosylation. We have compiled an extensive collection of glycosylated and non-glycosylated Asn-X-Thr/Ser sites and present a statistical study based on this data set. Our results indicate that non-glycosylated sites tend to be found more frequently towards the C termini of glycoproteins, and that proline residues in positions X and Y in the consensus Asn-X-Thr/Ser-Y strongly reduce the likelihood of N-linked glycosylation. Beyond this, there are no obvious local sequence features that seem to correlate with the absence or presence of N-linked glycosylation. These findings are discussed in terms of the prediction and engineering of glycosylation sites in secretory proteins.