SUMMARY1. Thirty‐four patients with three or more spontaneous abortions were studied. Among 193 recorded pregnancies, 150 had ended in spontaneous abortion, two in stillbirth, three in neonatal death, and only twenty‐nine in surviving infants.2. Analysis of variance and sib‐sib correlation showed that a given patient tended to miscarry at about the same time in different pregnancies. Spontaneous abortion was most common between the tenth and the twelfth week; there was a lesser peak between 20 and 26 weeks. There were twenty‐five patients in the early group, six in the later group, and three not easily classified.3. The sex ratio for the live births was 0–94.4. The sibships of the patients and their husbands were not smaller and did not have significantly higher abortion rates than the general population.5. None of the patients' matings were consanguineous. One husband's parents were first cousins.6. The blood group types Ms and MNs were slightly commoner than expected among the patients. Otherwise, blood group frequencies did not show any significant peculiarities. No excess of serologically incompatible matings was found.7. Chromosome analyses in leucocytes from peripheral blood were made for twenty‐two of the patients and eight husbands. The only abnormality detected was a large F‐chromosome in one husband.The authors are indebted to Prof. L. S. Penrose, F.R.S., for encouragement and advice; to Prof. W. C. W. Nixon, F.R.C.O.G., for permission to study the patients in the Obstetric Department, University College Hospital; to Miss Barbara J. Warland, for enlisting the patients' co‐operation; to Dr E. S. Shalom, University College Hospital Medical School, for performing the protein‐bound iodine determinations, to Dr E. B. Robson for serum analysis of patient 3 and to Miss Linda Gorman, for technical assistance.One of us (P. T. R.) was a Donner Foundation–National Research Council Fellow in Academic Medicine during this investigation and gratefully acknowledges their support.