In a pilot study, on a hospital-based series consisting of 285 type 1 and 282 type 2 patients with Diabetes Mellitus (DM) we compared the month-of-birth with the standard birth curve. In accordance with a previous investigation on 23,620 diabetics in the Netherlands, we found an excess of DM births in the first quarter of the year (p less than 0.005) and a deficiency of them during the last one. This excess corresponds with conceptions during the spring restoration of the ovulatory pattern, this deficiency with conceptions during its winter stabilization. Identical peaks and troughs have been found in month-of-birth studies of individuals with chromosomal anomalies and with anencephaly. Similarly, ovopathy--which we consider a common cause for multiple anomalies--can explain the high incidence of DM in Down's syndrome as well as in other chromosomal aberrations, and its association with unusual dermatoglyphics. Furthermore, the ovopathy concept appears in line with the consistently found maternal age and parity effect, the discordancy in one-egg twins and the distortion of HLA-DR phenotype distribution in IDDM multiplex families. Although our conclusions must be guarded because of sample bias and doubts concerning precise classification, we found that the configurations were stronger in the type 2 DM sample. Ovopathy might prove to be the crucial environmental factor in the causation of IDDM--searched for by many scholars--and a common cause for both types. The HLA-DR haplotypes might rather be the "trigger", influencing the course and type of the disease.(ABSTRACT TRUNCATED AT 250 WORDS)