In order to further characterize women with islet cell antibodies (ICA) at the diagnosis of gestational diabetes (GDM), we aimed to compare titres and persistence as well as B-cell function with those of ICA+ first-degree relatives of Type 1 (insulin-dependent) diabetic subjects. Titres at detection of ICA were compared between 69 women with GDM and 53 relatives. Persistence of ICA positivity was investigated in 33 ICA+ gestational diabetic women and 39 relatives (mean follow-up: 13 months). Assessment of the acute insulin response, through an intravenous glucose tolerance test (IVGTT), was carried out in 9 ICA+ women with previous GDM and normal oral glucose tolerance, and their results were compared to those of a control group (9 women) and a group of 12 adult ICA+ relatives. In comparison with first-degree relatives, women with GDM had a higher ICA prevalence, especially with titres of < 20 JDF, and a similar persistence at follow-up. Women with ICA at diagnosis of GDM and normal oral glucose tolerance after pregnancy showed a decreased insulin response to glucose as compared to the control group. Results of the IVGTT closely paralleled those from adult ICA+ relatives. It is concluded that ICA+ gestational diabetic women share with ICA+ first-degree relatives of Type 1 diabetic subjects metabolic and immunologic disturbances.