HAVE THE NATION'S older cities really regained their health? One could conclude as much from all the tales about an "urban renaissance" and surges of middle-class investment in formerly rundown neighborhoods -- a process that has produced the new mouthful word, "gentrification." In a recent article in Harper's magazine, journalist T.D. Allman even asserted that the "urban crisis" is over. That notion has been quickly embraced by many who, for various reasons, want to cut federal urban aid.

It would be lovely if such rosy forecasts were supported by the facts. However, the dramatic revivals in some parts of some cities are just that: highly visible but limited improvements that have not ended local governments' fiscal problems or reversed the long-term exodus of jobs and prosperity.

In response to Mr. Allman's piece, analysts at the Department of Housing and Urban Development have issued a more careful and sobering summary of the current condition of urban America. As staff writer Susanna McBee summed up in an op-ed article in Monday's paper, "some cities are doing better than they were," but the assertion that "cities on the whole are well off" is "fundamentally wrong." For instance, HUD found that the average income of families moving out of cities between 1975 and 1977 was $16,000, compared with $15,000 for those moving into town. The difference may not seem great but, with more families moving out than in, central cities overall lost more than $17 billion in family income during those few years alone. Whether or not one agrees with HUD's policies, such figures are a good corrective to blithe assumptions that urban problems are fading away.

A related dispute about displacement also shows the hazards of generalizing from fragmentary facts. Displacement of the poor is the harsh side of innercity redevelopment (or "gentrification," if you must). If you assume that the frenetic housing speculation and private rehabilitation in Washington and parts of other cities is typical, it figures both that cities are booming and that a lot of lower-income people are being uprooted nationwide. However, HUD has just reported to Congress that evidence of massive general displacement cannot be found. One survey concluded that around 500,000 people -- four percent of those who move each year -- do so involuntarily. That includes people displaced by private or public renewal and those evicted because their buildings are being abandoned or they have not paid their rent.

This report has come under some fire for seeming to minimize a serious problem. But HUD is not denying that poor people have been displaced in parts of Seattle, Portland, Washington and other cities. Again, the point is that urban change is not uniform, much less uniformly swift. As Miss McBee put it on Monday, "America's cities are not a monolith." That bolsters the case for giving cities even more leeway in using the federal aid available to them, so they can tailor public programs to suit their own particular problems of decline, stagnation and change.