The vitamin D status of 53 Caucasian infants aged 10-20 mo on a macrobiotic diet and 57 matched control infants on omnivorous diets was studied. In late summer (August-November) physical symptoms of rickets were present in 28% of the macrobiotic group; these infants had lower average plasma 25(OH)D concentrations (34.0 +/- 15.3 nmol/L) (mean +/- SD) than did the macrobiotic infants without such symptoms (49.7 +/- 21.9 nmol/L, p less than 0.02). Follow-up of a subsample of 25 macrobiotic infants in March-April revealed physical symptoms of rickets in 55% of the macrobiotic infants. All concentrations in blood were considerably below those in the preceding summer; the average 25(OH)D concentration was 12.3 +/- 4.3 nmol/L. Further analysis indicated that the low availability of calcium in the macrobiotic diet was an independent factor in causing the high prevalence of rickets in summer. Avoidance of milk products in combination with a high fiber intake may damage bone development in young children.